Checks if a package exists on PyPI prior to obtaining link count.

Fixes #4

The loop in main probably needs some refactoring. Feel free to alter output to taste.

py27: commands succeeded
py33: commands succeeded
flake8: commands succeeded
congratulations :)

$ pep438 requests
✓ requests: 0 links

$ pep438 requestsa
✗ requestsa: not found on PyPI