Refactor default sources.

This moves the default sources into the package and updates the search
paths so that rosdep searches for sources in /etc/ros, $ROS_HOME, and
within the package itself.
Use isdir or isfile instead of exists.
Add migration hook to remove old sources.
Include sources in package.
clean up egg-info on 'make clean'
Oops; clean up debug code.
Remove groovy and hydro gbpdistro lines.
Add dummy/deprecated init command
Implement searching for sources per discussion.
Note about gbpdistro on ROS distros after Fuerte.
Remove redundant if.
Remove old references to rosdep init
Merge pull request #242 from ahendrix/master

Move default sources into rosdep.