Allow forward slash

As I mentioned in #24, I think an option for escaping characters not required to be escaped by the JSON spec is premature, but if you think it should be there I can add it.

FWIW here's the relevant section from the spec:

> All Unicode characters may be placed within the
   quotation marks except for the characters that must be escaped:
   quotation mark, reverse solidus, and the control characters (U+0000
   through U+001F).