add boost depends to CMakeLists

All non-catkin things that we expose in our headers should be added to
the DEPENDS, so that packages which depend on our package will also
automatically link against it.

Also see: http://answers.ros.org/question/58498/what-is-the-purpose-of-catkin_depends/\#58593
Merge pull request #15 from mintar/add_boost_depends

add boost depends to CMakeLists