Add Simplecov to track test coverage

* Add Simplecov gem https://github.com/colszowka/simplecov
* `.gitignore` the new /coverage directory

Minor:
* Set Ruby version to 2.1.0 in Gemfile and `.ruby-version` file
* Remove comments from everywhere
* Sort lists alphabetically in groups
* Specify Ruby version in Gemfile

Ignore /coverage/ directory

Add Simplecov to spec_helper