Feature attribute fixes

- [ ] @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. 

So this file still shows up in this branch but it shows up as an empty file. Not sure if that's just the way github is displaying it or if its actually a git issue.