Escape \ and " in JSON strings

applescript-json's string encoding capability should be 100% compliant now. Nice!

As per the official standard ([ECMA-404](http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf), p 10):
> 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.