ENH: add ujson support in pandas.io.json

This is @wesm PR #3583 with this:

It builds now, and passes travis on py2 and py3, had 2 issues:

- clean was erasing the *.c files from ujson
- the module import didn't work because it was using the original init function

Converted to new io API: ``to_json`` / ``read_json``

Docs added