Initial integration with alternate content sources.
add docstirngs.
Fix download unit tests.
add alternate unit tests.
use os.path.basename()
Removed exta blank line.
refit alternate.Packages.download to use generator.
Merge branch 'master' into jortel-US2888
Better name.
Merge pull request #530 from pulp/jortel-US2888

Integrate yum importer w/ alteranate content sources