Added support for Linux kernel SPI spi-bcm2708 and TLC59711 PWM module

- Adjusted .gitignore to allow updates in packages/repositories.config. 
- Corrected three .csproj files with new <Reference> tags to the NuGet package directory.
- Refactoring of interop methods to Raspberry.IO.Interop
- New classes for managed/unmanaged memory and memory subsets
- Native SPI connection (userspace driver, device file /dev/spidev0.0)
- TLC59711 device driver (single device & cluster/chained device support)
- Added some NUnit tests
- Sample program for Adafruit's TLC59711 board