Fix sdist generation

[nils-werner]

This PR implements two things:

It adds all assets to the sdist zips by defining all files to be included in MANIFEST.in. By doing so

1. won't have to create each release ZIP manually
2. you may add SimpleCV to PyPI by running python setup.py sdist upload
3. Your users may install SimpleCV without using the URL by simply typing pip install simplecv

Also, I've fixed the install_requires keyword in setup.py. Before, the installation could not declare its dependencies properly, which in my case left svgwrite missing.