Merge pull request mavlink#158 from hamishwillee/master

Update Master to 0.15.0 attempt 2!

Merge pull request mavlink#156 from hamishwillee/release1_5updates

APIRef - update to 0.15.0

Merge pull request mavlink#149 from hamishwillee/id_attrate

ApiRef for Info:Identification and Offboard:AttitudeRate

Update in preparation for creating O.5 branch

Merge pull request mavlink#114 from hamishwillee/master

Update master with develop since v0.3.0

Update build.md

Make it clear how to select master vs develop branch, and that develop is used by default.