Option: Map URL is now disableable (#357)

Fixes #357.

Couple of notes:

- The option is reversed `omit_source_map_url` as opposed to `source_map_url` to make it a breaking non-breaking change (for instance, the current state of `node-sass` is working just fine with this change. later they can provide a CLI switch and the code to back in `sass.js` and `binding.cpp` to make use of this feature).

- I used C99's `stdbool` in `sass_interface.h`. With GCC's `std=c99` and VC12 (devenv2013), its supported and compiled well on Win7 (don't know about the others compilers). I saw @akhleung's old comment about lack of bool support in C. Let me know, if you guys have concerns regarding C99 (particularly `<stdbool.h>`), I will change it to `int`.