Feature attribute fixes

Features:

 - Deleted `common.rb` attributes file and created `default.rb` instead. According to https://docs.getchef.com/essentials_cookbook_attribute_files.html, `default.rb` is loaded first then the rest of the attribute files in alphabetical order. By moving the `common.rb` attributes into `default.rb` there is no longer a need to use `include_attribute "cassandra::common"`. This also prevents attributes from accidentally being overwritten.
- chefspec tests! Currently `67.65%` coverage across `centos 6.4` + `ubuntu 12.04` Simply run the following command: `bundle exec rspec --color --format documentation` in the root dir of the repo.
- Now when you install `dsc21`, the following attributes are set: `default[:cassandra][:setup_jna] = false` and `default[:cassandra][:setup_jamm] = true` defaulting to the `0.2.6` version of `jamm`
- Fixed issue where dir `/usr/share/java` wasn't being created.