Fixes #254

Fixes #252

Tries to fix #261

This ensures we end up with the correct capitalization of user's login
name, and gives the opportunity to the server to change the project name
without having us choke on it.

Fixes #263

 - performs a single API call instead of 3
 - simpler handling of parent repo data
 - less brittle implementation

Closes #257, references #146

The problem was that the dumb JSON generator simply dumped strings with
Ruby's `inspect` method, which on 1.8 generates escape sequences that
are incompatible with JSON.

Now strings are dumped simply as they are, with only a handful
characters escaped (double quote, backslash, whitespace).

Fixes #235

References #257, references #247

Fixes #245

References #235

This ensures that a Hub command can't accidentally kill tests.

When reading config, "insteadOf" substitution rules aren't applied.
Using `git remote -v` is better because it reports the URLs that would
actually be used by git.

This reverts the feature that supports detecting the GitHub URL among
multiple others made for #59. If you have multiple URLs configured for
the same remote, put the GitHub one first and you'll be fine.

Fixes #241

I tried to hack Sinatra to automatically serialize non-string response
bodies as JSON, but this is not straightforward due to sinatra/sinatra#512
and doesn't really work in a whole lot of edge-cases.

Introduced a simple `json` helper and now using that instead of `body`.

Fixes #238

References #216

References #216

Fixes #207

References #216

Fixes #208

fixes #218, fixes #194

Fixes #120

Enterprise installations don't have a separate subdomain for API
requests such as "api.github.com", but their API endpoints start with
"/api/v3" path.

Fixes #201