Portability Enhancements

This pull request encompasses the following changes:
 - Changed `Repo.find_scripts` to use `pkg_resources.find_distributions` to get the package metadata directly instead of executing Python in a subprocess, which was causing TypeError problems on Python 3.4 at least.
 - Changed the method of determining whether a command exists or not in `get-pipsi.py` to checking the contents of a generator that spits out everything in the PATH instead of looking for failures in Popen
 - pipsi now works on Windows:
   - All path specifiers in `get-pipsi.py` and `pipsi.py` are now portable
   - Added a compatibility wrapper for Windows that creates .bat file wrappers that act like symlinks in cases where symlinks cannot be created there. This allows this to probably work in Python 2.x on Windows, and definitely allows Python 3.x to work on Windows where the user does not have permissions to create symlinks, which appears to be the default.
   - Added a matching compatibility mode to `real_readlink` so that it can return the same information from those batch files