* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter

* build: [finishes #15372] implements generalized linter and travis job

* .travis, build: minor polishes, disable deadcode

This commit adds a build step to travis to auto-delete unstable archives older than
14 days (our regular release schedule) from Azure via ci.go purge.

The commit also pulls in the latest Azure storage code, also switching over from
the old import path (github.com/Azure/azure-sdk-for-go) to the new split one
(github.com/Azure/azure-storage-go).

Fixes #3792 by stripping debug symbols.

This ensures 'make test' finds all errors that remote CI would find.
Go 1.7 vet reports a false positive in package log, add a workaround.

This fixes a recent bug where 'make geth' built everything instead of
just geth.

We don't use the opencl build tag anymore.

We have decided to bump the requirement to Go 1.7 because it enables
subtests and allows dropping backwards-compatibility code. This is in
line with Go's support policy. Go 1.6 and earlier no longer receive
security updates.

The Android NDK was recently removed from gomobile, leading to our Android
builds failing. Starting from https://go-review.googlesource.com/#/c/35173/ ,
gomobile requires a locally installed NDK. This PR ensures that travis installs
that too before running the build steps.

NSIS has a default MAX_STR_LEN of 1024. If $ENV{PATH} is longer
the returned string is truncated to an empty string. Its then not
possible to distinguis between the variable not set or too long.
As a result the variable is set with the location where geth and/or
dev tools are installed. This may override any previous set values.

* build: modify the iOS namespace to iGeth (gomobile limitation)
* mobile: assign names to return types for ObjC wrapper
* mobile: use more expanded names for iOS/Swift API