Allow use of any pins by implementing Soft SPI

[ricardojlrufino]

Q	A
Bug fix?	no
New feature?	yes
Doc update?	yes
BC	yes
Deprecations?	no
Fixed tickets	#...

This change allows the use of any pin by implementing communication via software.

Due to the poor design of the SPI class, the only way I could do this integration was by creating a new define (MFRC522_SOFTSPI), which needs to be enabled in the file: MFRC522.h

I added a new example that demonstrates the use: DumpInfo_SoftSPI

Tests:

Q	A
DumpInfo	yes
rfid_write_personal_data	yes
rfid_read_personal_data	yes

This enables direct stacking of NANO and PRO MICRO arduinos, as in the example:

[ricardojlrufino]

This can be merged ?

[Rotzbua]

A software based SPI should be an extra library and not a part of a rfid library. Injectable SPI is available at the fork.