Merge lp://staging/~adeuring/charm-tools/check-maintainer-branch-owner into lp://staging/~charmers/charm-tools/stable

This branch adds a check for the maintainer name and the branch owner.

The main idea is that maintainer(s) as specified in metadata.yaml
should also be members of the team that owns the related Launchpad
branch.

I thought a bit if it makes sense to find the intended target
branch of the proofed branch: In theory, it is possible to look for
the parent branch, the "grandparent" branch etc until an LP branch
for a sourcepackage of the "charms" distribution is found. That _could_
be the the final merge target for the given branch, but that's not
always the case -- somebody might create a fork of another branch
with the intention to maintain this fork for a longer time -- and this
is probably the main use case for the new check: To ensure that the
maintainer mentioned in metadata.yaml has write access to the LP
branch of the charm.

Another option would have been to find a push branch on LP and to check
for merge proposals, to eventually find the "right" target. But that is
also not trivial -- and it does not work if the proofed branch has not
yet been pushed to Launchpad.

I other words: Lots of cumbersome "guess work" with limited success
probabilty.

So I took the most simple approach: The user must explicitly
specify the merge target of the given branch.

Currently, the proofer checks if the name of the main charm directory
matches the charm name. This does not make sense for branches, so this
required some changes how the directory name is handled in function run().

I found the distinction between charm_name (a directory path) and
charm_base_name a bit confusing, so I renamed charm_name to charm_path
and charm_basename to charm_name.

It seems that we do not have any infrastructure for unit tests of
functions that access the LP web API, so I wrote also a little mock
for the LP API.