Support for JSON

Extracts string fields from a JSON document because hey, who doesn't love recursion? Suitable for text analysis of MongoDB dumps via mongoexport command. Output does not maintain / enforce any order of fields in an object, so the result should be used in bag-of-words modelling- n-gram algos beware. Also includes a small doc revision. All tests are passing on travis: https://travis-ci.org/anthonygarvan/textract. 

Also, the md5sum seems to be broken for the snow_fall.html example on Ubuntu 13.10 (fails for me locally but passes on Travis). Using OS utilities for this seems overkill - any reason not to use python's hashlib? 
This is an interesting way of handling this. Maybe we should rename all of the parser modules to something like `{{ext}}_parser` to avoid conflicts with globally installed packages.

We run into this problem in the `docx` and `pptx` modules too and we use the [utils.non_local_import function](https://github.com/deanmalmgren/textract/blob/master/textract/parsers/docx.py#L7) to deal with it. While clever, I'm not terribly happy with that implementation and think you've got a better way of handling this. I'll make some changes to how those are incorporated after merging.