Add RSpec to the gem (not only the dummy app)

The Cucumber features ensure the gem behaves as expected, however
the increasing complexity of the gem internals makes me wish to
be able to write some unit tests.

The idea is to describe the lib/ components behaviour with RSpec,
each on their own; and to ensure they work well together with Cucumber.

That should also make contributions easier: the features testing
is becoming a serious bottleneck and the complexity of the Cucumber
setup doesn't help at all. Some unit tests should help reducing that
complexity in the long term. Here we go! :)
Add example spec for a name generator method

See https://github.com/gonzalo-bulnes/simple_token_authentication/pull/90#discussion_r16754902
Added support for Rails-API, made email attribute name customizable
Added missing configuration attribute
Fixed little documentation mistake
Fix remove Rails API support
Fix missing config key
Temporary revert "Fix remove Rails API support"

This reverts commit 32f72262f44d8678fbc6eab6efa75e164be4afbb.

The Rails API support should be removed before merging because
it is off-topic, however, it may be useful during developement.
If you need it, revert this commit, I'll clean that before merging.
Fix remove Rails API support (fixup)

I forgot half the fix.
Temporary revert "Fix remove Rails API support (fixup)"

This reverts commit 981f9fc2a1ada2b4ebf1a46b0bff6c92534142e0.

This commit exists for the same reason the previous does:
  see Temporary revert "Fix remove Rails API support"
Merge remote-tracking branch 'origin/spike-add-custom-authentication-identifier-option'

Conflicts:
	Rakefile
	spec/spec_helper.rb