Remove the coverage rake task

RCov does not work on Ruby 1.9.x
Add simplecov as a dependency

Simplecov works with Ruby 1.9.x for generating test coverage
Add SimpleCov to the unit tests
add the coverage directory to .gitignore
filter out test files in coverage results
Merge pull request #765 from mattr-/fix-coverage

Fix coverage
Update history to reflect merge of #765.
Refine CONTRIBUTE with more detail and better formatting.