I2C support

[Lukelectro]

Q	A
Bug fix?	no
New feature?	yes #240
Doc update?	yes
BC breaks?	no
Deprecations?	not aware of any, tested with DumpInfo sketch and SPI reader
Fixed tickets	#240

Because I2C uses less wires, it can be useful sometimes.
RC522 support HS I2C, but the software doesn't as of yet. See the commented-out SwitchToHighSpeedI2C(); and the RC522 datasheet.

Tested I2C functionality with a small green board labeled P/N:0021 RC522 I2C V1.1

Tested BC with a SPI reader and the DumpInfo Sketch.

[Lukelectro]

Please let me know if I need to make changes or if should do the pull request against another branch then master.

[Rotzbua]

Hi @Lukelectro,

thanks for your pull request. I review your pull request, I think there is some more work to do in the init function. Never the less I think polymorphism is not a good solution. A split between communication (SPI, I2C) and management (mfrc522) logic would be a preferable solution.
I did not found a solution which do not break backwards compatibility and do not consume a lot of flash storage.

I would prefer a fork by you as solution.

Best wishes