Ensure accurate encoding/decoding of big and small floats

ujson has trouble with very big, and especially very small, floats:

esnme/ultrajson#69
esnme/ultrajson#83
esnme/ultrajson#90

From: https://github.com/pydata/pandas/pull/1263#issuecomment-20024006

    >>> import ujson
    >>> ujson.dumps(1e-40)
    '0.0'
    >>> ujson.dumps(1e-40, double_precision=17)
    '0.0'

I believe this makes ujson inappropriate for inclusion in pandas. 

This pull requests adds tests that ujson should, but won't, completely pass. I don't feel yet sufficiently familiar with pandas' design to substitute in (e.g.) simplejson.