Mercurial configuration methods

We'll need the equivalent of the following HgGitRepository methods in the Rails app:

• set_hgrc -> SetRepositoryManagedHgrc or HgSetManagedConfig
• get_hgrc -> GetRepositoryManagedHgrc or HgGetManagedConfig
• hg_config_item_bool-> HgGetConfigItemBool
• create_hg_repository except the actual hg init part, which is taken care of by #58 (closed) -> HgInitRepositoryConfig or HgInitConfig

This is obviously a divergence from the Gitaly protocol, so that we'd better speak of the HGitaly protocol from this point. We can either make these new methods in RepositoryService or make a new service. In the former case, maybe it's obvious that it is for a given repository, so we may not need to include that in the method names.

The bulk of implementation should be done in py-heptapod, and exposed both as a CLI command (for the Rails app) and in HGitaly, hence avoiding a code duplication that would certainly be harmful.

Opinions welcome, especially about the naming matters.