Pull request for branch 'merge-usbinterface'

Dear all,
some maintenance on the existing USB interface code I have worked on:

I have merged the USB-interface code that has been extended with RPC by Beat (?) and was located in src/BasePixel/usb.* into the original USB-interface code residing in src/interface/USBInterface*; since the CUSB class which talks to the FTDI drivers (either libftdi or libftd2xx depending on user choice at compile time) inherits from CRpcIo (defined in rpc_io.h), I have moved the RPC code from src/BasePixel into src/interface;

Furthermore, all methods of the libftdi variant of USBInterface behave now the as in the libftd2xx implementation and offer the same functionality.

The code has been tested on Ubuntu 12.10 with both FTDI drivers on a DTB with single-ROC module attached and works so far.

I have also cleaned up and updated the documentation to reflect the option w.r.t. the choice of FTDI driver.