ENH: JSON

revised argument structure for ``read_json`` to control dtype conversions, which are all on by default:

- convert_axes : if you for some reason want to turn off dtype conversion on the axes (only really necessary if you have string-like numbers)
- dtype : now accepts a dict of name -> dtype for specific conversions, or True to try to coerce all
- convert_dates : default True (in conjunction with ``keep_default_dates`` determines which columns to attempt date conversion)

DOC updates for all