Commit the source-map library.
Implement basic source maps. Closes #1252.
Add check for keep_llvm_debug + minor touchups.
Add test for source maps.

Tweak behavior of post_build; the `post2` hook now expects a function.
Ensure line numbers are the same in HTML.

Chrome and FF seem to be able to handle the extra empty lines, but no
harm making things more accurate. One thing less to worry about when
debugging.
Merge branch 'source-maps' of github.com:int3/emscripten into incoming
Merge branch 'incoming' of github.com:kripken/emscripten into incoming
comment on EXPORT_ALL
raise an error if no functions were left to process