added mouse input

The event system has been reworked to allow the detection of mouse
events as well as key presses.
Xterm, rxvt and X10 emulated escape codes are supported, they are
enabled and disabled by sending the right escape codes when creating a
RawTerminal.

To allow for byte manipulation, which was necessary to implement those
features, the backend iterator has been changed from chars() to bytes()
(with specific treatment of unicode sequences), making the whole crate
not require nightly rustc.