Add some existing modules

Common:
 - Parity generators and checkers
 - Fixed priority arbitor
 - 7seg LED controller
 - 4ch Multiplexer
 - UART controller

RAM:
 - Simulation models
 - ISSI IS61LV25616AL-10T 10ns SRAM for Spartan-3 FPGA Starter Kit board

Sample FPGA projects for ISE w/ Spartan-3 FPGA Starter Kit board:
 - 7seg LED
 - UART
 - RAM