Fix display bug when group title is a number

Due to the way MarkupAdminDataTable works, the link is not generated
properly if the group title is a number.
Adding pagination in group list

Limit the number of results, otherwise the page takes a very long
time to load with a lot of groups.
Adding group description (#31)

Adding a new field "group_description"
Adding a new column in list views
Adding a new input when creating the group to enter the description
Merge pull request #29 from rlanvin/groupnumeric

Fix display bug when group title is a number
Merge pull request #30 from rlanvin/pagination

Adding pagination in group list
merged changes made by @rlanvin (description to group list)
show group title on edit view (not editable though)