Prioritize default branches above others

[sdboyer]

go get relies, at times sort of implicitly, on having the upstream repository determine which branch should be grabbed on initial clone...at least in cases like git, where branches are multiplexed and not expressed directly in the path. (Also the case for hg, not the case for bzr or svn).

In order to remain maximally similar to go get, we need to have these branches always sort higher than other branches in version queues (where they are otherwise alpha-sorted). This guarantees that the solver will try them first, which amounts to the being the same as go get when no semver tags are available.

Implementing this symbolically shouldn't be very difficult - an extra field on branchVersion to indicate whether it's a default branch or not. What's slightly trickier - or at least possibly more costly wrt network requests - is figuring out what that default branch is in ListVersions().

[sdboyer]

...crap, also need to change the sort order for version types to put branches above plain versions. ...i think that's the right call? probably.

EDIT: fixed this in #98