Implement OG-USA version of taxcalc/parameters.py and taxcalc/policy.py

[hdoupe]

This PR is based off of the work started by @jdebacker in PSLmodels#354.

Files added:

1. parametersbase.py -- A stripped down version of taxcalc/parameters.py. I removed most of the inflation indexing logic and array expansion logic since that doesn't seem to be as important for OG-USA. Further, stripping the class down into the core parts helped me understand the code better. The indexing and array extension logic could be added back.
2. pb_api.py -- A stripped down version of taxcalc/policy.py. Most of the indexing and other logic that isn't relative to OG-USA was removed. Also, two new methods are implemented:
   a) ogusa_set_default_vals -- sets cheaply calculated parameters that are difficult to represent in a JSON file
   b) estimate_parameters -- sets expensively calculated parameters such as those estimated for the tax function
3. tests/test_pbapi.py -- tests for the pb_api.py module. More tests need to be added. In addition to covering new OG-USA logic, applicable tests from the Tax-Calculator suite should probably also be ported over.

The work so far serves mostly as a proof-of-concept. Many of the more complex parts of the Tax-Calculator logic were removed to get this code stood up quickly. The parametersbase.py code will probably better maintained and tested more thoroughly if it is used by both Tax-Calculator and OG-USA. So, I may take another crack at implementing a pb_api.py on top of the taxcalc/parameters.py class instead of extensively changing the taxcalc/parameters.py class.

cc @jdebacker @martinholmer

[hdoupe]

Whoops. Just realized that I opened this PR on the wrong branch. I meant to do Jason's pb_api branch. I'll close this and open another PR.