adds instrumentation and coverage reporting functionality using istanbul.

- integrates javascript instrumentation into sprockets (filtering spec/helper files.)
- adds reporting of coverage information in console reporter.
- adds cli options for coverage.
- updates readme to include configuration information.
adjusts coverage output.
brings our teabag_env up to date.
adjusts specs to not mess with Teabag.configuration as much.
builds the javascripts.
remove spec_helper from the filter (so it can also get instrumented).
fixes the specs I just broke.
adds coverage section to the readme.
minor adjustments. [ci skip]
update readme to be less redundant. [ci skip]
adds the ability to specify files in a suite that shouldn't be instrumented.

- allows removing helpers, etc. from coverage stats.
removes the find_asset override because it was causing sprockets caching to fail.
fixes how instrumentation works.

- doesn't break caching.
- is simpler.
updates readme, configuration, and initializer templates for better no_coverage support.
adjusts executable call from instrument.
fixes up instrumentation further to make it less intrusive.
Merge pull request #31 from modeset/coverage

adds instrumentation functionality using istanbul.