Custom alias and environment variable additions

Allows for the addition (through attributes) of custom aliases and environment variables to the bashrc file.

Also adds testing through kitchen and rspec, and some refactoring of variables in the recipe into a helper module.
You should use a shared path for the ec2 pem so it doesn't need to be copied/linked in each cookbook project.
```
ENV['EC2_SSH_KEY_PATH'] ||= File.expand_path('~/.ssh/id_rsa_kitchen_ec2')
```