Remove Rails version >= 3 conditional

We don't support Rails <= 3.0 anyway, so this conditional is useless
add ctags file to gitignore
Move ActivityLogging DM compatibility check to adapter

This is another step to decouple adapters from the rest of the library. In the
future mechanism of checking for compatibility can be extended, so that plugins
may add their own verifications, but right now it's not necessary.
Merge pull request #583 from arnvald/move_datamapper_exceptions_to_adapter

Move ActivityLogging DM compatibility check to adapter
Enable JRuby support for AR

Right now this is just an experiment, I'm enabling JRuby support for
ActiveRecord (with `allow_failures` option on Travis), we'll see how it goes.
Add facebook display setting in the initializer
temporarily set jruby tests only for rails4
Cleaning up
Separated adapters from model #581
Cleaning up unused integration spec
Add jruby builds for mongoid and mongo_mapper (Rails 4.x)
Memoization of sorcery_adaptor
Minor
Fixed DM current_users method
Adapter capability and proper calls to adapter.find
Use adapter find method instead of ORM's

DataMapper `find` method returns enumerator. To return a single record we need
to use our own `find_by_id` method which always returns one result
Fixed missing methods in adapters
Merge pull request #590 from NoamB/separated-adapters

Separated adapters from model #581
Merge pull request #588 from arnvald/experimental_jruby_support

Enable JRuby support for AR
Merge pull request #600 from rogercampos/initializer_fb_display_setting

Add facebook display setting in the initializer