Refactor boilerplate

Move Coveralls dev dependency to gemspec

Add some dev dependencies

 * Guard for RSpec
 * Pry + goodies
 * _(considered adding RuboCop as well, but postponed)_

Enrich CI testing

 * test Ruby 2.0.0 and 2.1.1 compatibility, besides 1.9.3
 * speed-up Travis by caching the gems we’ve specified
 * enable email notification when specs fail
 * spec is the default RSpec task

Use rake to run specs

Minor refactorings