repository.Log: add alternatives for commit traversal order

[ilius]

The new option Order to repo.Log allows customizing the order of commits.
Specially because the current algorithm is not compatible with git log, the current algorithm use a stack (Depth-first Search?), while git log uses timestamp of commit (committer time).

So if we want to have the same order as git log we specify Order: git.LogOrderCommitterTime.
In the CommitterTime implementation, the stack of iterators is replaced with a BinaryHeap of commits (to avoid sorting the commits every time a commit is added). Also start property is removed (we start with a heap of length 1) so the code is simpler and cleaner.

For the BinaryHeap we use this library: https://github.com/emirpasic/gods

The default behavior (commits order) of repo.Log is unchanged.

[ilius]

The failing test does not seem to be related to my changes

/home/travis/gopath/src/gopkg.in/src-d/go-git.v4/plumbing/transport/test/upload_pack.go:55:
    c.Assert(err, Equals, transport.ErrRepositoryNotFound)
... obtained *errors.errorString = &errors.errorString{s:"unexpected EOF"} ("unexpected EOF")
... expected *errors.errorString = &errors.errorString{s:"repository not found"} ("repository not found")