Issues with supporting Cargo

[steveklabnik]

Related to #8

Now that #57 has landed, we can have more than one project. But there's a problem: cargo's commits aren't being sorted into releases properly. The logs show commits being assigned multiple times, and then finally to master, since it's the last release that gets assigned to. My current guess is that the way that we assign commits to releases relies on Rust's branching model, which cargo doesn't share. More investigation needed.

[steveklabnik]

We are going to have to do custom, per-project stuff here; let's close this in favor of the more general #8.