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
