Implement surrogate pairs in the encoder.

[rpgoldman]

Original PR comment:

Transform surrogate pairs characters into appropriate JSON escape sequence.

Reference:
http://unicodebook.readthedocs.io/unicode_encodings.html#utf-16-surrogate-pairs

[ghard]

Would the hold-up for one of these not being merged be that there are no test cases written?

[ghard]

Also if encoding using surrogate pairs is likely to cause backwards-compatibility issues with people using this library, we could add a special variable to turn it on, and maybe an accompanying macro like with-non-bmp-char-surrogate-pairs?

[rpgoldman]

A test would really help, yes, and I think you are right to think it should have a switch to enable or disable this. I will have to have a quick look at the docs and then I will try to suggest a method for switching that has the same flavor as the other configuration mechanisms.