Background image support, PygcurseTextbox single char fix, Transparent font rendering

New backgroundimage and foregroundimage properties allow you to set an image behind and in front of the drawn Pygcurse window. These are not affected by tint, and do not change the cell colors either.

Fix: PygcurseTextbox border='?' (single char) crash.

Renders fonts transparently if their bgcolor is set to ERASECOLOR. 

A sample of these: http://imgur.com/RWh0S5s