Adding support for ACL permissions per module

This pull request allows to set permissions per module in the Joomla Module Manager. So besides configuring the com_modules general permissions you can configure the permissions per module. 

You can use this for example to: 
- allow access to com_modules, but only editing a selection of modules
- allow access to com_modules and edit all modules expect one (or more)
- disable state changes for certain modules
- etc..

## Notes
This patch doesn't include the correct sample data for the assets table. Happy to add that once the pull request is successfully tested. So to make sure the modules have a reference in the #__assets I suggest to simply open and save a module. 

## Known issues (input welcome!)
#### JavaScript issue:
- create new user group and add test user to this group
- allow access to module manager and leave other com_modules actions as not set 
- allow edit action for one of the module
- login with test user and edit the module
- click on the save button, a JavaScript error will occur: "TypeError: b is null"

## JoomlaCode Tracker Item
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=28638

## Testers welcome!
Testers are much welcome, let me know if you have any questions / feedback. Thanks in advance!