Make git-buildpackage work on both trusty and precise

Intended to address https://github.com/mikeferguson/buildbot-ros/issues/33

Basically, git-buildpackage needs different command line arguments on trusty than it did on precise. This is made more difficult because it is the git-buildpackage version on the slave that matters, not on the master. To deal with this, I've added wrapper scripts for building source and binary debs which get downloaded to the slave and run. When they are run, they check the version of git-buildpackage and call it with the appropriate arguments.

I've tested this on trusty, but I don't have a precise machine to test on. Can someone try it out? If something doesn't work, it should be a relatively simple matter to change the commands called in the wrapper script when running on precise.