MR return to home and landing sequencing

[peabody124]

So encouraged by Oso Grande I rewrote the vtol follower to allow executing multiple step commands (https://github.com/TauLabs/TauLabs/issues/1012) and used that to write fly home and land at home sequences.

We tested it at our Houston meetup this weekend, and I thought it was pretty reasonable https://vimeo.com/79603584. There is a more complete writeup on my blog http://buildandcrash.blogspot.com/2013/11/return-to-home-and-landing.html but basically it works pretty well IMO. A few changes should be done:

1. better altitude control settings by default

2. update the set up instructions to include calibrating the z-accel

3. make it ascend in place before RTH