test Runner.check_inputs
external_process - minimal convenience wrapper for subprocess
external_process.Result: add string conversion for easier debugging
Runner: add virtualenv related functions

  - run_in_virtualenv(self, cmdspec)
  - install_package(self, package_spec, index_server_url)
  - make_virtualenv(self, index_server_url=None)
PEP8
tests: add path2url - helps to make a local, directory based index_server_url