New hostgroup features

This pull request adds two new features to the hostgroups view

1. adds a hosts tab on the edit page of a hostgroup to view a list of hosts belonging to the hostgroup, and easily access their links
![screen shot 2014-08-21 at 3 37 52 pm](https://cloud.githubusercontent.com/assets/7310311/4173292/a1e3f97c-3557-11e4-8a46-36873e80e796.png)


2. adds a new display action - this is linked to on the hostgroup index page for each hostgroup IF the user does not have edit permissions. This essentially renders the same page as the edit action would, but runs a Javascript function to disable all edit fields and remove buttons so the user can only view details, not edit them, as a user with only view hostgroup permissions could only see a list of hostgroups earlier, not the details of each individual hostgroup. 
![screen shot 2014-09-05 at 4 51 15 pm](https://cloud.githubusercontent.com/assets/7310311/4173289/93de19e8-3557-11e4-9bff-b255ff8d27a3.png)

Please if ```host.os.present?``` when checking if a relation object is present.