add lcb_urldecode

This adds a function to decode strings encoded in 'percent encoding'.
This is needed for the URI handling within the library.

Change-Id: Ic46740678e0975360c0c015c6b93cb82dfd35134
Reviewed-on: http://review.couchbase.org/38103
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>