Updates documentation

Apart from updating the documentation, this patch also introduces the
<tt>rake doc</tt> task, for creating/(re)building the documentation.

Note, that this might take a while til run.
Display test coverage with simplecov
Expose character advancements on character model

This exposes the character’s advancements on the character model,
through the character levels, character class(es), character class
level(s) and lastly the advancements.

This is a shorthand, and supports filtering on level and character
class.
Use Fuubar for rspec progress