diff_drive: rename cmd_vel_old_threshold to cmd_vel_timeout

The default is now 0.5s instead of 1.0s
diff_drive: update unit tests
diff_drive: add unit test for cmd_vel_timeout
diff_drive: fix comments
Merge branch 'hydro-devel' into cmd-vel-timeout

Conflicts:
	diff_drive_controller/CMakeLists.txt
	diff_drive_controller/src/diff_drive_controller.cpp
Merge pull request #21 from po1/cmd-vel-timeout

Diff drive: rename cmd_vel_old_threshold to cmd_vel_timeout and add unit test