Require missing dependency ActionPack

... which provides ActionController

See https://github.com/rails/rails/tree/4-1-stable/actionpack/lib
Refactor ensure adapters are loaded only when needed

ActiveRecord is a runtime dependency; the ActiveRecordAdapter
will be loaded as usual and this commit introduces no changes
in the gem behaviour.

Fixes #97
Minor add adapter interface spec

The shared specs for the 'adapter' interface provide a better
help if they spot the methods which miss an override.

Adding this spec makes the shared specs no longer usable with
the `Adapter` module itself. Some more caution will be needed
to ensure both the module and the shared specs stay in sync, but
the module should change less often than new adapters will be added.
Minor refactor tidy up TokenAuthenticationHandler

Fix circular dependency (!)
Minor fix harmless variables shadowing in dynamically defined methods

Remove forgotten hack related to the deprecated Cucumber scenarios
See #104
Require ActiveRecord as development dependency

Fix Travis CI build, force `--no-deployment` installation

As long as an ORM/ODM/OxM is available, there should be
no obligation to install ActiveRecord.

About the Travis issue, see:
https://github.com/gonzalo-bulnes/simple_token_authentication/ \
commit/5c04e13c44379d95b95c1affcdb85177cf6188e3 and
https://stackoverflow.com/q/26221540/3767264
Refactor database queries for use with Mongoid, ActiveRecord

The `exists?` method has distinct meaning in Mongoid and AR.
See http://mongoid.org/en/mongoid/docs/querying.html#queries

The `find_by_email`, `find_by` too.
Add Mongoid support

Add Mongoid as development dependency
Add, enable MongoidAdapter

Thanks to @mediatainment who contributed the "Mongoid support" part
of this this commit a long time before it became production-ready and
who litterally ensured the support for users in the meantime.
See https://github.com/mediatainment/simple_token_authentication/ \
commit/6b30379ebed0f277487b63f3ec1e29129d598196

Fixes #38
Fix files definition error in gemspec

The files definition should not have been modified.
Fixes 7ca0499b572c1ad470b258b0e7df125925bfd80e
Merge branch 'add-mongoid-support-using-adapter'