fallback python-based .ps extraction

[deanmalmgren]

The .ps parser currently uses pstotext to extract content from postscript files. This is a great starting point, but it might be nice to replicate the behavior of the .pdf parser and have a pure python fallback method to make textract usable across platforms. From a minute of googling around, it looks like others have started down this path:

• https://github.com/haxwithaxe/py-ps-parser
• https://github.com/zeffii/PostScriptParser

I'm not sure if it makes more sense to roll our own or just use these other packages to extract text in the right way (I have a slight bias for this approach), but I thought I'd throw this issue together in case it inspires ideas or contributions from others.