Investigate using source repositories instead of release repositories

[gavanderhoorn]

Some bugs appear to be reported for repositories that -- at the time of the report -- did not have releases registered in ros/rosdistro. This leads to problems such as #5.

rosinstall_generator supports arguments / options that make it use source repositories instead of the release repositories which could be used to generate .rosinstall files in these cases.

As rosinstall_generator_tm.sh is essentially just a wrapper around rosinstall_generator, it should be fairly easy to use the source repository functionality, but we'd have to check whether the tricks we perform with the cache work for those entries as well.