Rebase from 'upstream'
Fix root cmake for building as ros 3-rd party library
Fix pc.in template
Add cmake find_package config

Thanks for urdfdom package for examples.
Add cmake rules for pymavlink installation.
Add install rule for package.xml
Fix python invocation for mavgen (python2)
Use external script for pymavlink installation (as catkin).

Some code from catkin build system used to fix installation process.