Allow overriding the mapping from source file to source map.

This is done with the new retrieveSourceMap option to sourcemap.install.

This is useful if you have another way of finding your source maps (and can avoid sync reads).

Also, allow source maps to not have an URL, and don't write to a global named mapSourcePosition.
Merge pull request #18 from meteor/retrieve-source-map

Allow overriding the mapping from source file to source map.