Prefer 'pip' to install packages and only fallback to 'easy_install' if 'pip' is not available: 'easy_install' seems to require elevated privileges on some versions of MS Windows while 'pip' does not).
Add option to 'virtual.bootstrap' task to prefer 'easy_install' over 'pip' if both are installed: 'pip' doesn't install binary eggs, but 'easy_install' does, so especially on MS Windows there is a common and legitimate use case to prefer 'easy_install' over 'pip'. Assorted bug fixes.
Test Python 3.3 support