content-disposition support

Here is my cleaned up version of #27 up for consideration.

 1. I removed some of the white space change to make the PR easier to read
 2. added a test
 3. support both unadorned filenames (as given by github) and quoted filenames as specified in the spec

The regexen that I have employed probably are not exhaustive, but they probably address most of the actual use cases out there in practice.  Note that there is a conflict with #24, so maybe that PR could add some abstraction so that we don't have to special case every interaction with the response object/hash.

I vote for the inclusion of this PR: aye.