Auto merge of #1931 - thirtythreeforty:printinfo, r=alexcrichton

Auto merge of #1931 - thirtythreeforty:printinfo, r=alexcrichton

Compare #1621.  Like that pull request, this one solves #984.

I have rewritten most of @pyfisch's logic to account for @alexcrichton's comments.  Specifically, this code will correctly handle adding/removing multiple versions of one package, as well as several packages with different sources, but the same name.

The output looks like this:

```
[georgev@desertvoice cargo]$ cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Updating libc v0.1.8 -> v0.1.10
    Updating memchr v0.1.3 -> v0.1.5
    Updating num v0.1.26 -> v0.1.27
    Updating rand v0.3.9 -> v0.3.10
    Updating rustc-serialize v0.3.15 -> v0.3.16
```

Comments welcome.

r? @alexcrichton