Update git usage page with advice on pull requests etc

We could update http://biopython.org/wiki/GitUsage to be much shorter (refer to more external resources at GitHub or elsewhere rather than duplicate things) (_update: perhaps not, see below_), and concentrate on for Biopython specific topics like our development model (and link to http://biopython.org/wiki/Contributing or a new page specifically on coding styles, testing, etc).

For example, since we've tried to continue a single-branch model, most of our pull requests have been rebased and fast-forward merged (giving a linear history). To facilitate this, it can be helpful for the contributor to sometimes rebase and update their pull request (`git fetch origin` then ``git rebase origin/master` and `git push your_fork your_branch --force`), although often the person merging the PR will do this at the command line.