Added plugin class tracking
Pulled in some Locomotive code and tests

Refactoring Locomotive and the locomotive_plugins gem so that all of the
functionality which isn't specific to Locomotive is in the gem instead

Some specs are commented out and still need to be fixed so that they
pass
Set up framework for putting plugin object into the context when drops, tags, and filters are called
Fixed add_plugin_to_context helper
Cleaned up specs
Changed before_filters to ActiveModel callbacks
Fixed a comment
Removed DB Models. Locomotive will isolate the database
Added new explanation for Database Models to README
Removed old before_filters
Removed old DBModel methods
Removed pre-initialization block
Removed more old before_filter functionality
Updated README and removed plugin registration
Fixed some lingering references to plugin registration
Fixed plugin tracking