File input and output

Even though this branch is labeled JSON I/O, it also includes the following changes:

* addition of (un)pickling functionality: now the PUF file gets read in as a dictionary without us having to convert it to such from a pandas DataFrame
* an option of writing the output of taxcalc as a JSON file (I've also kept csv output as an option, albeit commented the running of that function out)
* Plan X and (optionally) Plan Y are now read in as JSON files (I can attach Plan X if you'd like)
* ``translation.py`` now looks up the relevant variables either from the Plan X dictionary or the PUF dictionary

@feenberg let me know if you have absolutely any questions about the interface. If you click on the tab "Files Changed" above this message on the right, you should see the actual modifications I made to the code. Moreover, you should be able to comment on individual lines: just hover your mouse over one and click on the blue "+" icon that appears slightly to the left. I hope using this more will bring concreteness to our discussions.