Add support for embedding source files into the source map

Using sourcesContent property as per the [source maps spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?hl=en_US&pli=1&pli=1#heading=h.lmz475t4mvbx). This PR resolves #212.

Be aware that I'm not a C developer, so I'm really making guesses here although I have enlisted the help of someone who does know C. I *think* it's correct, but I don't have the knowledge to write tests or really validate that what I've done is correct.

I've added integration for this [into node-sass](https://github.com/aexmachina/node-sass/compare/source-map-sources) but all I get is a segfault :( Happy to pursue it further if you have some suggestions but I feel that it might be better for someone with some clues about C to take it from here :)

FYI, the JSON code is [taken from CCAN](git.ozlabs.org/?p=ccan;a=tree;f=ccan/json).