This introduces support for Subversion and Mercurial repositories hosted
on Codebase HQ. We now go through the Codebase API for all repository
validation and file fetching, which gives us the ability to work with
all supported repository types.

There is one bug in their API that we're encountering and working
around, which is that their file contents API is returning contents with
the final trailing newline removed. This breaks patching, and in order
to work around it, we need to add that newline back in. This could
conceivably cause other problems (and we're having to make an educated
guess about the type of newline to insert), but it will solve the common
case until Codebase fixes the problem (which has been reported).

There's no support yet for post-commit review, as their API lacks
commits pagination and the ability to get a diff for a commit.

Unit tests pass.
Tested each type of repository by hand. I was able to validate the

repository configuration, post changes, and view the diffs.