Wrap git_blob_is_binary native method as Blob.IsBinary

Hi,

I just implemented the [`git_blob_is_binary`](https://github.com/libgit2/libgit2/blob/development/src/blob.c#L300-310) method. I took the implementation of `Blob.Size` as an example, using the `GitObjectLazyGroup` to call the native method lazily.

I would have like to test the true case as well, but it seems there is no binary blob in the test fixture repo. Am I right?