Bug fix for pip 1.4 compatibility (workaround for global state)

Related to pypa/pip#1084:
  https://github.com/pypa/pip/issues/1084
Cleanup pip command line argument parsing
Bug fix for pip 1.4 compatibility (workaround for PreviousBuildDirError)
Fix code indentation of recent changes
Merge pull request #14: Support for pip 1.4 (just released)

See pull request #14 on GitHub:
  https://github.com/paylogic/pip-accel/pull/14
Make pip 1.4 support compatible with automated tests