fix(connection): set token expiration date for GCE

A response from the GCE metadata server responds with data in a
different format from that of the GAPIToken() request. Instead of
looking for a property that doesn't exist `token_expires`, we now
calculate the expiration timestamp from `expires_in`.

For more, see [Authenticating from Google Compute Engine][auth].

[auth]:https://developers.google.com/compute/docs/authentication#applications

Fixes #212
feat(docs): versioned docs

'versions.txt' contains the available versions one per line.
The json files containing the docs for for each version are stored in
'docs/json/<version>/'.
'npm run docs' creates the json files in docs/json/development,
and version.txt contains the 'development' versions, to allow to serve
the docs locally in an easy way (using for example static).
Update home.html

fixed branding issues
Merge pull request #222 from bgoldy/master

Update home.html
docs(contributing): document git commit conventions

To introduce contributors to our git conventions, our CONTRIBUTING.md
file now links to the AngularJS contributing document.
feat(docs): rename 'development' version 'master'
Merge pull request #220 from silvolu/versioned-docs

feat(docs): versioned docs
Merge pull request #224 from stephenplusplus/commit-guidelines

docs(contributing): document git commit conventions
fix(connection): allow more connection callbacks

Node's default limit for max listeners on an event emitter is at a low
10. We are using an event emitter internally in the Connection class to
handle queueing callbacks for the token retrieval event. The default of
10 is now lifted to unlimited, so the developer doesn't have to worry
about overriding the default themselves.

Fixes #223
docs(contributing): remove AngularJS confusion

Eliminate potential confusion caused by referring to the AngularJS repo.
We also prefer the use of a `common` scope over `app` for library-wide
changes.

Resolves #224
remove commit formatting requirements - resolves #226
fix headline grammar

-`Unit tests` and `Regression Tests`
+`Unit Tests` and `Regression Tests`
build: release on git tag

After a tag is created:
- a new coverage report is generated
- a new version of the docs is generated and committed to gh-pages
- a new version is deployed to npm
docs: fix examples, style optionals
(storage) document write options. fixes #161
Merge pull request #228 from silvolu/release-on-tag

build: release on git tag
datastore: transaction: only commit once
Merge pull request #229 from stephenplusplus/fix-docs

docs: fix examples, style optionals
docs: Rename Google Cloud to Google Cloud Platform.
Merge pull request #235 from rakyll/gcp

docs: Rename Google Cloud to Google Cloud Platform