Load From File message

On Linux when the user presses cancel on the "Load from File"
dialog, was getting an error message saying the app could not
open the file.

Also fixed a spelling error in ApmPlaneLevel.cc.
Update README

Update the readme to refer to the diydrones location.
Merge branch 'master' of https://github.com/kellyschrock/apm_planner
Merge branch 'doc_update'