Fix memory leak by removing dynamic class

The proxy class only needs to be defined once per scope declaration
rather than at the time of calling the instance methods.  This resulted
in the anonymous class sticking around even after the instance was deleted.

Thanks to Senthil V S for the analysis of the problem at:

http://siliconsenthil.in/blog/2013/01/19/validation-scopes-leaks-memory/
Tighten up all_scopes code
Remove Jeweler and add Gemfile for development
Convert to pure minitest

Shoulda is just needless bloat here.  Straight, simple, to-the-point tests
Specify Rails 3.0 requirement

Also added Gemfile comments to indicate how to test 3.0.x and 3.1.x
versions which are supported as well.
changelog updates
Use VERSION file in gemspec and bump it to 0.5.0
Improve tests a bit
No longer needed in Ruby 1.9.x
Update README
Version fix and ruby version requirement