Fixed #14 - Proper detection if an entry file name already exists.

The plugin was looking in the wrong place to see if the file name
was already being used. This has been fixed now. A new UUID will be
selected if the file name already exists.
Added better uuid generation.

Now using tcjennings' uuid4 generator
(https://github.com/tcjennings/LUA-RFC-4122-UUID-Generator).
Merge branch 'bug-fix'