Expand the list of valid tarball content types

The existing community site has no validation on content type, so the
following content types exist in the database:

* application/x-gzip
* application/gzip
* application/octet-stream
* application/x-tar
* application/x-compressed-tar
* application/x-gtar
* application/x-bzip2
* application/gzipped-tar
* application/x-compressed
* application/download
* application/x-gtar-compressed
* application/zip
* application/x-bzip
* application/x-zip-compressed
* application/cap
* application/x-tar-gz

We can impose validation going forward, but in order to import the
existing community site data, we'll need to relax our constraints.
Merge pull request #318 from opscode/relax_tarball_validation

Loosen tarball content type validation