Fixes source map generating (#586)
Adds json library by Joseph A. Adams
Windows: Add guard for snprintf.
Also fixes the warning for unsigned char*.
Adds base 64 encoding library

http://sourceforge.net/projects/libb64
Adds `add_source` helper function
Implements source map embedding

- embed sources content inside source map
- embed source map inside sourceMappingURL
Excludes external libraries from code coverage

They provide far more code than we actually use.
And we certainly don't want to test all that code!
Reverts back to using longhand options in travi-ci!
Fixes some minor clang warnings in sass.cpp.
Merge pull request #591 from mgreter/source-map-sources

[WIP] Implements embedding source contents in source maps