FEATURE: Add group(s) membership support to Bulk Invite feature

This PR adds group(s) membership support in Bulk Invite feature. Admin can now invite user to single/multiple group(s) by adding semicolon separated group names in second column, eg:

![screen shot 2014-06-25 at 04 07 55](https://cloud.githubusercontent.com/assets/5732281/3379342/4bfc5c48-fbf0-11e3-809e-84646a798dfd.png)

If the group name is incorrect and doesn't exist in database, a PM notification will be sent to Admin:

![screen shot 2014-06-25 at 03 19 38](https://cloud.githubusercontent.com/assets/5732281/3379347/74e19bb4-fbf0-11e3-878f-5015be9eca59.png)

The group name(s) must be entered in second column, in comma separated format, else user will not be added in group.

TODO:
* Update [how-to](https://meta.discourse.org/t/send-bulk-invites/16468) to reflect this change.