[WIP] Use symlinks internally.

Prior to this change, the total time to merge was dependent upon the actual file sizes of each merged file (since we were copying them), and the total number of files. 

##### Non-Windows

Changing this to use symlinks as high up the chain as possible instead removes the file size and count from consideration.

Initial build from 2705ms to 306ms.
Rebuilds from 3505ms to 246ms.

##### Windows

Stays essentially the same as it was previously.  This means that we are still copying all files for every merge. There were a few small fixes that *might* result in a performance increase, but it will be largely the same.

* Requires https://github.com/joliss/node-walk-sync/pull/2 to be used throughout the ecosystem. Specifically, with `broccoli-filter`.

Closes #10.