Objective-C Mavlink generator (mavgen_objc.py)

It's an Objective-C implementation of Mavlink!

This pull request has the necessary changes to mavgen.py and the addition of mavgen_objc.py that support the creation of an Objective-C wrapper around the C APIs. This makes it much more pleasant to use Mavlink in iOS and Mac applications.

Note: This commit depends on the two other pull request I've had open (#127 & #128). The diff included in this pull request also includes the commits for those other pull requests as well, since it won't work without them. I recommend resolving those other pull requests before merging this one.