Implemented CustomClassLoading, as per #124. The ClassLoading code is much cleaner than before. I have added a few tests to test it, but these should be expanded further to really stress out corner cases.

Note that I have added some class files as data files. I did this because they are consumed as data by the ClassLoaders, and because I needed to be able to test cases where some of the class files are missing.
Merge branch 'master' of https://github.com/int3/doppio