Update the Open Babel API in Avogadro

[psavery]

Several changes have been made to the Open Babel API
which results in errors in the Avogadro compilation with
an updated Open Babel. This fixes our API issues.

[psavery]

FYI: this is currently incomplete. It DOES compile. But it has a few issues. Two issues I have noticed so far:

1. It isn't automatically adding hydrogens correctly.
2. The camera seems to think the atoms are bigger than they are. It zooms out a ton when you draw close to the edges.

For Avogadro 1.3, do you want to use the most up-to-date Open Babel (so that we need to update the Open Babel API)? If you don't, I can just abandon this. But let me know if you do, and I'll try to fix those bugs.

[ghutchis]

I'm unsure about using the latest OB API, since it's not backwards compatible. I'm thinking it will be declared as OB-3.0. It's certainly a lot faster for many tasks, but I'm not sure how quickly it will be packaged by Linux distributions.

Let's keep this branch here - I appreciate the work on this.