new createrepo api

Part of all the changes in createrepo was to enable a simpler api to be
used by calling python programs rather than just invoking createrepo on
the commandline. The api is pretty straightforward. I'll give a simple
introduction:

import createrepo
mdconf = createrepo.MetaDataConfig()
mdconf.database = True # make the sqlite databases
mdconf.directory = '/home/skvidal/tmp/f' # directory to run on
md = createrepo.MetaDataGenerator(config_obj=mdconf)
md.doPkgMetadata()
md.doRepoMetadata()
md.doFinalMove()

That's really it.

It's not complicated but it gives a good starting point for people to do
custom population of their repodata. I dunno if there's a lot of use for
this but it needed to be done anyway, if only to clean up the code.

Thanks,
-sv