Merge lp://staging/~julian-edwards/launchpad/metadata-type-bug-595038 into lp://staging/launchpad

= Summary =
Add meta-data custom upload type for use by the Software Center.

== Implementation details ==
This branch defines a new custom upload type, META_DATA, which is defined by a
file's section being "meta-data".

The file's format is of no concern to Soyuz, it simply lives in the librarian
and is published to an area outside of the repo so that if the repo is
private, the meta-data is still accessible.

== Tests ==
bin/test -cvvt publishing-meta-data-files.txt -t meta-data-custom-files.txt

== Demo and Q/A ==
We need to make a source package with a meta-data custom file attached and
upload. It should get accepted as normal and published appropriately.

= Launchpad lint =

Checking for conflicts. and issues in doctests and templates.
Running jslint, xmllint, pyflakes, and pylint.
Using normal rules.

Linting changed files:
  lib/lp/soyuz/model/queue.py
  lib/lp/soyuz/interfaces/queue.py
  lib/lp/archivepublisher/tests/publishing-meta-data-files.txt
  lib/lp/archiveuploader/nascentuploadfile.py
  lib/lp/archiveuploader/tests/meta-data-custom-files.txt