Escape \ and " in JSON strings

As per the official standard (ECMA-404):
> All characters may be placed within the quotation marks except for the
> characters that must be escaped: quotation mark (U+0022), reverse solidus
> (U+005C), and the control characters U+0000 to U+001F.
Merge pull request #3 from winny-/master

Escape \ and " in JSON strings