Add simplejson-compatible parameters

[alecf]

In particular:

dumps needs at least:

• indent which is a number of spaces to indent
• sort_keys - whether to sort dict keys in the output (for predictable serialization)
• default - which is a function that gets called on anything that's not normally json-encodable, return value is used as the string value

not sure loads() needs anything.

[mikeal]

API compatibility with simplejson would be ideal:

http://docs.python.org/library/json.html#json.dump

[alecf]

A clarification on indent:

indent=None (the default) means "dont' indent, make JSON the minimum size (i.e. no extra spaces)
indent=N, where N is an integer means "use N spaces to indent, with newlines, etc"
indent=0 means "no indenting, but still use newlines" (i.e. consistent with N)

[rtyler]

Support for indent on dump/dumps added with SHA: adeeb98