Proper file naming for DataMapper (datamapper -> data_mapper)

The problem with previous naming is that if we call `"datamapper".classify` it
will return `Datamapper` instead of `DataMapper`.

As I'm trying to get rid of coupling Sorcery with ORM adapters, this has become
an issue in my attempts to implement generic adapter setup
Merge pull request #578 from arnvald/data_mapper_file_naming

Proper file naming for DataMapper (datamapper -> data_mapper)
Update #increment in Mongoid to allow 3.x and 4.x

The syntax of increment method changed from 3.x to 4.x. This commit makes sure
it will work in both of them
Test different Mongoid versions

We support Mongoid 2, 3 and we're aiming to support Mongoid 4.
Changes in gemfiles allow us to check all of them (but not all Rails-Mongoid
versions combinations!)
Fix gemfiles to test properly agains Mongoid 4 and Rails 4 and 4.1
add gemfile for Mongoid 3
Merge pull request #575 from arnvald/mongoid_4

Mongoid 4
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