Added new modules to library

- gc_i2c_slave     -- generic I2C slave to be used with a processor or tied to a
                      Wishbone interface
- gc_glitch_filter -- glitch filter with selectable number of taps
- wb_i2c_bridge    -- I2C bridge implementing the protocol defined with ELMA
                      for monitoring VME crates
- wb_xil_multiboot -- module that accesses the Spartan-6 configuration logic
                      for reconfiguring the FPGA using MultiBoot

Doc files for each of these modules can be found in the doc/ folder.

Signed-off-by: Theodor Stana <t.stana@cern.ch>
Signed-off-by: Tomasz Włostowski <tomasz.wlostowski@cern.ch>