GPS support

I tried to implement GPS support, but as stated in the commit message for log2cvs the lat/lon values are bogus. I suspect I screwed up the read32() function but an endiannes conversion bug might also be to blame.

When using this with qgroundcontrol altitude flops around between 10m and -540m, but log2cvs reports a stead '827', so that is likely a multiwii bug.

So this isn't really fit for merging yet, but I wanted to get it out there so more people can look at it.