Fixes https://github.com/prometheus/prometheus/issues/481

While doing so, clean up and fix a few other things:

- Fix `go vet` warnings (@fabxc to blame ;).

- Fix a racey problem with unarchiving: Whenever we unarchive a
  series, we essentially want to do something with it. However, until
  we have done something with it, it appears like a series that is
  ready to be archived or even purged. So e.g. it would be ignored
  during checkpointing. With this fix, we always load the chunkDescs
  upon unarchiving. This is wasteful if we only want to add a new
  sample to an archived time series, but the (presumably more common)
  case where we access an archived time series in a query doesn't
  become more expensive.

- The change above streamlined the getOrCreateSeries ond
  newMemorySeries flow. Also, the modTime is now always set correctly.

- Fix the leveldb-backed implementation of KeyValueStore.Delete. It
  had the wrong behavior of still returning true, nil if a
  non-existing key has been passed in.

storage: Fix float->uint conversions on some compilers.

See https://github.com/prometheus/prometheus/issues/887, which will at
least be partially fixed by this.

From the spec https://golang.org/ref/spec#Conversions:

"In all non-constant conversions involving floating-point or complex
values, if the result type cannot represent the value the conversion
succeeds but the result value is implementation-dependent."

This ended up setting the converted values to 0 on Debian's Go 1.4.2
compiler, at least on 32-bit Debians.

web: Fix regression in .Path

.Path is documented as removing /consoles/,
recent changes added in a leading / which broke
the provided console templates menu system.

retrieval: avoid race conditions

web/js: fix display error for vectors in console

Export build information as metric.

There is more build information that could be put into labels, but
Version, Revision, and Branch seemed like the right level of detail to
me.

New dockerfile using alpine-glibc base image and make

Fix the race condition targets in the Makefile.

Remove race_condition_run as the original run target has been removed, too.

Fix the race_condition_binary target.

config: hide authentication credentials in String() output

promql: marshal sample pairs to JSON tuples.

resolve rule files relative to config file.

Add tests for "globalURL" template function.

Consolidate external reachability flags into one.

Besides fixing https://github.com/prometheus/prometheus/issues/805 by
making the entire externally reachable server URL configurable, this
adds tests for the "globalURL" template function and makes it easier to
test other such functions in the future.

This breaks the `web.Hostname` flag (and introduces `web.external-url`).
This flag is likely only used by few users, so I hope that's
justifiable.

Fixes https://github.com/prometheus/prometheus/issues/805

api/v1: fix response format tests

Add circleci yaml for Dockerfile test build

api/v1: fix Content-Type in response

retrieval: Handle serverset node not existing.

This stops configuration loading hanging if
the Znode doesn't exist, and retries until the
node does exist.

Preserve alert state, cleanup rules/