Add Heart Beat Module

This adds an optional pulsing block to the output; both the passed
through signal and the recorded image.

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.
Update gitignore

I did a build using ISE 14.4 on my Ubuntu machine. These ignores give me
a clean tree.