Add ability for grouped layers to be exclusive.

Simple modification that allows to specify exclusive layer groups by adding the
name of the group to to options.exclusiveGroups.
This resolves ismyrnow/Leaflet.groupedlayercontrol#2
and can be used to gain the functionality discussed in Leaflet/Leaflet#593
Merge pull request #15 from kleingeist/gh-pages

Add ability for grouped layers to be exclusive.