Fix compilation on OS X.

 - without this linking fails on OS X (10.9)
 - prefer find_package over pkg-config, since pkg-config does not link
   transitive dependencies (see https://bitbucket.org/osrf/gazebo/issue/1202)
Merge pull request #30 from NikolausDemmel/fix-osx

Fix compilation on OS X.
update changelog
0.3.3