Expose handler classes in top level import

[pllim]

Instead of

 from baldrick.github.github_api import PullRequestHandler

would like to have

 from baldrick import PullRequestHandler

[Cadair]

I think I would prefer

from baldrick.github import PullRequestHandler

Just from a namespace perspective, but either way works.

[pllim]

Unless you plan to expand to baldrick.gitlab, baldrick.bitbucket, etc, I don't see the point for more typing. 😉