New data type JSON_BSTRING for binary strings.

This string output type replaces all non printable bytes (00-07, 0b,
0e-1f, 7f-ff) to "\u00xx" sequences. This allows json decoders to
reconstruct binary string without interpreting binary sequences as
Unicode characters.
Merge pull request #13 from fln/master

New data type JSON_BSTRING for binary strings