Test coverage by rspec-puppet and puppetlabs_spec_helper integration

Hi guys,

i added some tests to your cassandra module. PR includes:

 - Switch to puppetlabs_spec_helper
 - 91 rspec tests to cover all classes and parameter validation
 - Added missing dependency to puppetlabs/apt
 - Updated Gemfile to track test depencies

Cheers :beers: 

Daniel