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