Refactor mockup generate to load external gem generators

This change makes it possible to load additional generators by subclassing HtmlMockup::Generators::Base
Add travisci integration
Add ruby language to travis.yml
Test with rake
Add Gemfile and ignore Gemfile.lock
Add rake to gem list for travisci
Add yiucompressor to gem dependecy
Fix tests against new Thor >= 0.18
Allow failures for jruby and rubinius

Since we have never supported these versions, we can allow_failures there.
And we are green on travis.ci [ci skip]
Fallback to register function for generators

While previously we relied on the inherited function, I removed this since it wasn't possible
to read out the arguments and description of the class, making decorating it for the help text
very hard.
Add generator for generators
Readme typo
Remove unused testfile
Refactor CLI into separate Thor::Groups and fix tests accordingly
Remove deprecated extractor method from cli commands. Use release instead
The new generator should just be a Thor::Group instead of Cli::Command as there is no project yet
Do not require pry in testing
Merge pull request #17 from DigitPaint/feature-generators

Refactor CLI and CLI/generators