Adds a table3 parser

As discussed in https://github.com/sunlightlabs/crosslaws/pull/21, this moves @wpli's Table 3 parser into this project. It has its own README and `requirements.txt`.

/cc @apendleton @alexengler

It'd be quite neat to produce some data that cross-linked the existing output of this project (the JSON "spine" of the Code) with the legislative history of codification.