Quadrotor

Added velocity_control to MikrocopterControl class, which was added to
work with joystick control. Takes forward and lateral speeds and
combines them to set pitch and roll.

Added ncurses for key input. It can now publish at a rapid rate because
the key input is non-blocking, and after it exits the terminal is not
screwy, but it's not an ideal solution because ROS_INFO no longer works.

Encapsulated the stuff in nav_lib into a class, MikrokopterControl.

Created nav_lib.cpp/h, with functions forward, backward, left, right,
and stop to set the mikrokopter's movement direction.
Replaced code in wasd_nav.cpp's main with forward(pub) and stop(pub)
from nav_lib.cpp.
Added nav_lib.cpp to CMakeLists.txt