Cull the list of described tasks

Some of this noise really is noise and is unlikely to be useful to
anyone. I removed the descriptions of file tasks that exist seemingly just
to support the functions of other tasks. I removed the descriptions for
tasks that are most likely to be used via a more general task.

Also refactored to move code out of the rake file, and split the
descriptions into separate yml files based on their namespace. This will
make it easier to adapt to changing how the gem works (opt in to certain
descriptions? add require to your rakefile rather than a railtie?)
moving forward.
Add test task to project rakefile

I can't figure out how to give this gem proper tests, so for now running
`rake test` will run `rake -T` in the dummy app. This might actually be
exactly what is desired.
Prepare for v0.9.1