Extended the evaluator_spec.rb test suite.

We are now able to check if our index suggestions are really useful.
The test suite queries MongoDB for query performance stats and checks
if our indexes improve them.
Enhanced index suggestions, fixed a few bugs.

Proper indexes are now suggested for sorting purposes.
Evaluating existing indexes was also improved.
A few minor bugs were discovered and fixed.
Coding style improvements

Conformed to the Github Ruby Styleguide.
Removed constants from EfficiencyResult and IndexResult classes.
Bug fix: expect explain[scanAndOrder] to be false

Somehow explain["scanAndOrder"] returns false instead of nil on the server,
this commit is fixing this issue.
Removed the side effects of ensureIndexQuality

Dropping the newly created index inside ensureIndexQuality method.
Merge pull request #57 from jonasz/work

Extended test suite, enhanced index suggestions, conformed to the style guidelines