Option: Map URL is now disableable (#357)

[am11]

Fixes #357.

Couple of notes:

• The option is reversed omit_source_map_url as opposed to source_map_url to make it a 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 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.

[mgreter]

I have a regression with this merge in my unit tests (Perl Binding CSS-Sass). Since this merge libsass will always include the sourceMappingURL if omit_source_map_url is not set. I wonder if this makes sense if source_map_file is not set to anything at all?

Currently appended to the resulting css:

/*# sourceMappingURL= */'

Changing

if (!omit_source_map_url) output += format_source_mapping_url(source_map_file);

to

if (source_map_file != "" && !omit_source_map_url) {
  output += format_source_mapping_url(source_map_file);
}

fixes my unit tests