SPARK-1623: Use File objects instead of String's in HTTPBroadcast

This seems strictly better, and I think it's justified only the grounds of
clean-up. It might also fix issues with path conversions, but I haven't
yet isolated any instance of that happening.

/cc @srowen @tdas

Author: Patrick Wendell <pwendell@gmail.com>

Closes #749 from pwendell/broadcast-cleanup and squashes the following commits:

d6d54f2 [Patrick Wendell] SPARK-1623: Use File objects instead of string's in HTTPBroadcast