gallery.py: Rework headers to use an OrderedDict

gallery.py: Rework headers to use an OrderedDict

This makes it easier to manipulate and access headers (e.g. to extract
the charset).  Also:

* Use CRLF line endings for headers [1], instead of our old LF.
* Encode text/ content to bytes (other content should already be in
  bytes) [2].  The encoding used for text/ content is configurable.

[1]: http://tools.ietf.org/html/rfc2616#section-6.1
[2]: http://www.python.org/dev/peps/pep-0333/#unicode-issues