Added Mike Ragalie to contributors list
Updated change log
Support streaming file transfers (issue #12) (issue #26)

This commit modifies the `DiskFileSystem` interface to support
streaming file transfers, and implements streaming transfers as the
default. Interface changes are as follows:

* DiskFileSystem#write now takes an `Ftpd::Stream` object instead of
  an `String`.
* DiskFileSystem#read expects to receive a block and yields an `IO`
  object to the block.
Update benchmarks
Update Changelog
Version bump to 0.17.0