Include missing discount source files

For v0.1.1, I'm running into compilation issues in my project. it's missing `basename.c` and `github_flavoured.c` in the podspec.

~~~
Undefined symbols for architecture x86_64:
  "_gfm_string", referenced from:
      -[GHMarkdownParser HTMLStringFromMarkdownString:] in libPods.a(GHMarkdownParser.o)
  "_mkd_basename", referenced from:
      -[GHMarkdownParser HTMLStringFromMarkdownString:] in libPods.a(GHMarkdownParser.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
~~~