qmake install now working under Linux

Running "qmake install" will build APM Planner and install it on the user's system.

Using the PREFIX=/some_dir/usr in the command will tell qmake to install APM Planner in the specified directory (default is /usr).

(edit on pull request #93 to correct package name to "APM Planner 2" rather than "APM Planner")