Wiring up existing source maps of transpiled codes

When `generateSourceMaps` option is enabled, r.js now detects if the module to be concatenated already declares a source map. It tries to load the existing source map, and translates line numbers of all mappings in that source map to those of the final output.

This fixes most of #470, and works well as long as `optimize` option is set to `none`.  Some work still remains to pass a correct `--in-source-map` option to UglifyJS when also uglifying.  Currently, UglifyJS ignores r.js's carefully generated source map producing a bogus one if `optimize` option is set to `uglify2`.

Used https://github.com/lydell/source-map-url (v0.2.0) for detecting sourceMappingURL= comments from the code.