Heart Beat Module Added

This module causes a small block of pixels at the lower right of the
screen to altenate between black and white.
Changes to firmware to add Heart Beat Module

The size of pulsing block can be changed by changing HB_length and
HB_width in hdl/misc/image_selector.vhd
Heart beat can be switched on/off using switch 0 or by sending "SH"
via CDC. To achieve this hdl/misc/controller.vhd and hdl/hdmi2usb.vhd
has been modified.
heart_beat.vhd add to .prj