Preserve folder structure for source-mapped files.

[colinbate]

Currently the path information other than the filename is lost from the sourcemaps which makes it difficult to work with them. This change sets the correct path for the source maps.

[xdave]

I can confirm the issue here. I have several "components" in their own directory with an index.js file as the entry point for them. IF there's a problem in one of these files, the console error in the browser links me to the first index.js it finds, not the one I need to see the error in.

[colinbate]

@xdave If you need something in the mean time, you can use https://www.npmjs.com/package/babel6-brunch which is this repo including PRs #26, #28, #29 and adding .babelrc importing.

[goshacmd]

@paulmillr @es128 should we merge all these PRs?