Use gmatch instead of gfind on lua5.2

This is needed for lua5.2 support which is used by the newest awesome (version 3.5). 

Note that It might be a good idea to provide a tag of the current code because some people will not move to the new version asap

Best, Nikola
Fixed #11 - using only 'Desktop Entry' group.

Actually, it will read all groups in separate tables. But only 'Desktop
Entry' will be used.
Merge branch 'patch-1' into issue-12

* patch-1:
  Use gmatch instead of gfind on lua5.2