Add support for Assembla Git Browser

For our project, we use Assembla's git repo and a local Jenkins. It would be really nice to have a browser class for the git plugin so we can reference diffs on our builds. I'm not very Java literate, but the code is very well written and documented so it was easy to get something together. Please let me know if I've made any mistakes here (I probably did, but I'd be psyched if I didn't). For diffs, Assembla does provide a "Compare feature to compare branches, commits, tags, etc, but it doesn't appear to be working all that well atm, and since getDiffLink() explicitly only compares the referenced commit and the preceeding commit, making the getChangeSet() call as seen should be appropriate as that page is already such a diff. However, getDiffLink() seems to want the diff for a specific file, and Assembla does not provide such a page (to the best of my knowledge), and instead presents the diff for the whole commit. If this is an inappropriate implementation of getDiffLink(), please disregard it or suggest an alternative. It would be great to get this feature into the update center repo so we can use it on our Jenkins. Thank you!