Feature attribute fixes

- [x] @mannytoledo 
- [ ] @chrishenry 

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 in alphabetical order. By moving the 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`
- Now when you install `dsc21`, `default[:cassandra][:setup_jna] = false` and `default[:cassandra][:setup_jamm] = true` defaulting to the `0.2.6` version of `jamm`
- Fixed issue where `/usr/share/java` wasn't being installed.