Refactor simplestreams prior to adding tools

Refactor simplestreams prior to adding tools

We need to be able to process juju tools simplestreams
metadata. To make this easy to add, the existing code
needed to besplit into generic and type specific parts.
Most of the functionality is provided by the framework.
The type specifc parts involve defining a struct to
receive the data and a function to do the filtering. 
The imagemetadata specific artifacts are provided. The
next branch will add the tools specific artifacts.

https://code.launchpad.net/~wallyworld/juju-core/refactor-simplestreams/+merge/178878

(do not edit description out of merge proposal)

[wallyworld, 2013-08-07 04:47:07]

Please take a look.

[wallyworld, 2013-08-12 03:10:02]

Please take a look.

[jameinel, 2013-08-13 13:44:04]

This looks mostly like just splitting out some bits into a different package,
and then refactoring all of those names to the new package and making some local
funcs into exported funcs.

If there are bits that are more involved that you want me to review the logic
of, I'm happy to give them a closer glance, but overall

LGTM

[fwereade, 2013-08-13 15:06:01]

Sorry, I "reviewed" this last week and never mailed it. LGTM, thanks.