ENH: json default_handler param

[Komnomnomnom]

This adds a default_handler param to to_json, which if passed must be a callable which takes one argument (the object to convert) and returns a serialisable object. The default_handler will only be used if an object cannot otherwise be serialised.

Note right now the JSON serialiser has direct handling for:

• Python basic types bool, int, long, float.
• Python containers dict, list, tuple and set.
• Python byte and unicode strings.
• Python decimal.
• Python datetime and date.
• Python None.
• Numpy arrays.
• Numpy scalars
• NaN and NaT
• Pandas Index, Series, DataFrame and Timestamp
• any subclasses of the above.

If an object is not recognised the fallback behaviour with this PR would be:

1. if a toDict method is defined by the unrecognised object that
   will be called and its returned dict will be JSON serialised.
2. if a default_handler has been passed to to_json that will
   be called to convert the object.
3. otherwise an attempt is made to convert the object to a dict by
   iterating over its attributes.

[jreback]

didn't this have an original issue? (or was that on the time serializatino?)

if not, just reference this issue in the release notes

passed on windows?

[jreback]

thanks.....(I updated the release notes)...and confirmed windows...