Adds support for anthrocon_plugin

Updates the vagrant install file to pull down anthrocon_plugin from
GitHub and add it as a plugin. It will alternatively use a local copy if
you have it cloned inside your ubersystem/ folder.
Merge pull request #3 from Anthrocon-Reg/add_ac_plugin

Adds support for anthrocon_plugin
Merge pull request #4 from Anthrocon-Reg/master

Pick up vagnrant script update