Add i2c function to read i2c device register

Hello

thank you very much for your C# i2c library. It works out of the box and I am using it. I enjoy running .NET assemblies created with VS Studio on my Windows host and finally excute them on Raspberry via Mono.

About this pull request:

Some i2c devices are internally organised by a set of Registers. Therefore I have added and tested the function readRegister().
The function requires the i2c bus device address and the Register address. It Returns the Content of the Register.

It may be useful for other users too.

It is used in Rpi.LCD_Display.NET to read the inputs attached to the MPC23017 port expander

best regards
Carsten