Needless class_eval

This was doing nothing dynamic, and the eval is wastefully repeated if
there are multiple calls to `acts_as_taggable_on` with a context.

The helper for these methods is already a regular instance method not
used anywhere else, so the `tag_types.empty?` guard wasn't exactly a
huge win in saving namespace pollution.
Unused variables
Whitespace :innocent:
Redundant include silliness

Know an Ruby module. This stuff confuses and misguides newcomers.

http://yehudakatz.com/2009/11/12/better-ruby-idioms/
De-ugly line continuations
Merge pull request #412 from ches/cleanup

[Misc] Code cleanup, refactoring