Add further documentation

* When to use stately
* Design Goals
* Example: using Stately with ActiveRecord
[#1] Fix stately_machine class var error
Add rake dependency

To avoid error with Ruby 2.0 in which running `bundle exec rake` yield

/lib/bundler/rubygems_integration.rb:214:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)

[Closes #2]
Add TravisCI

[Closes #3]
Merge pull request #3 from joneslee85/add-rake-into-gemfile

Add rake dependency
Merge pull request #5 from joneslee85/issue-3

Add TravisCI
satisfy travis