added fix to mac os x + virtualenv - and fixed test_basic.py to install distribute when using --user option

Another try at fixing #32 and #161. Use git fetch instead of git pull (which fetches and merges), before checking out the wanted commmit.

made it easy test framework delete tmp folders - e.g. pip-test-cache and TestPipEnvironment.root_path

Added option to disable prompting for input. Also added ability to interactively ask for username and/or password in case a request requires authentication.