Fixed #84. sites dropdown list is now sorted by url in add-site-to-group.
Fixed #85. user drop-down list in add-user-to-group is sorted.
Add filter and sort options to users page in admin. See #87.
Added filter and sort options to sites in admin. See #87.
Added filter and sort options to groups in admin. See #87.
Added filter and sort options to plans in admin. See #87.
Added sort and filter option to plugins. See #87.
Merge pull request #89 from yeukhon/issue84-87-sorting

Add flter and sorting to admin pages. Fixed #87.