Updated Gem description & added code coverage support.

Added a script to run all tests (useful for the coverage support).
Updated the travis-ci configuration for code coverage.
Fixed the travis.yml script.
Will run coverage only for Ruby 2.1.0
Removed 2.1.0 from the general build.
Modified the default Rake task to invoke coverage conditionally.

The environment variable COVERAGE (with any value) will trigger the code
coverage test; else the standard unit tests will be run.
Added a platform dependency for coveralls.
Removed the version requirement for rubysl.
Updated the changelog.
Merge branch 'br0.9.2'