gazebo_plugins: fix linkedit issues

Note: other linkedit errors were fixed upstream
in gazebo
gazebo_plugins: replace deprecated boost function

This is related to this gazebo issue:

https://bitbucket.org/osrf/gazebo/issue/581/boost-shared_-_cast-are-deprecated-removed
gazebo_ros: fixed missing dependency on TinyXML
Merge pull request #100 from ros-simulation/fix_osx

Fixes found while building on OS X