RPM Support

Steps to Reproduce:

1. create a repository with url "https://repos.fedorapeople.org/pulp/pulp/fixtures/rpm-with-sha-512/" and immediate download policy
2. sync the repository with --validate set to tue
3. check the rpm count in the repo

Download succeeded: https://repos.fedorapeople.org/pulp/pulp/fixtures/rpm-with-sha-512/zebra-0.1-2.noarch.rpm.
ERROR: (8377-95616) Listener error on event: succeeded
ERROR: (8377-95616) Traceback (most recent call last):
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp/server/content/sources/event.py", line 39, in __call__
ERROR: (8377-95616)     listener.on_event(self)
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp/server/content/sources/event.py", line 135, in on_event
ERROR: (8377-95616)     method(event.request)
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp_rpm/plugins/importers/yum/repomd/alternate.py", line 119, in on_succeeded
ERROR: (8377-95616)     self.content_listener.download_succeeded(report)
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp_rpm/plugins/importers/yum/listener.py", line 195, in download_succeeded
ERROR: (8377-95616)     super(RPMListener, self).download_succeeded(report)
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp_rpm/plugins/importers/yum/listener.py", line 86, in download_succeeded
ERROR: (8377-95616)     self._verify_checksum(unit, report.destination)
ERROR: (8377-95616)   File "/usr/lib/python2.7/site-packages/pulp_rpm/plugins/importers/yum/listener.py", line 150, in _verify_checksum
ERROR: (8377-95616)     if sums[unit.checksumtype] != unit.checksum:
ERROR: (8377-95616) KeyError: u'sha512'