errata_tool should support batches

[phracek]

Sometime we need to specify batches for an advisory.
In that order, new file called batches.py has to be created and support it.

https://errata.devel.redhat.com/developer-guide/api-http-api.html#api-batches

AC:

• errata_tool is able to add batches into advisory
• tests covers batches

[ktdreyer]

@yazug @fuzzball81 I think you guys are doing some work with batches right?

[yazug]

we are but currently only using the default behavior or having to rely on RCM/pgm to update

[kpfleming]

I'm a PgM working on a tool which needs to harvest batch-related data for errata, so I'd really be happy to see batch support in this package. #199 seems to be on the right track, at least for batch object support; is there anything I can do to help get that merged and released?