Allow to set crystall clock other than 16MHZ

[orenergy]

This will allow this library to work with modules with MCP2515+TJA1080+8MHz oscillator crystall.
This request depends on request at arduino-canhacker project

[autowp]

Looks great.
What you think about change type of canClock to enum like CAN_SPEED enum to prevent using of other values than 8, 16, 20?

Are you update corresponding examples, README.md?

[orenergy]

Didn't touched README and examples.
Ideally there's no need to change anything if you have module with 16MHZ (it's default for compatibility), but I'll write couple of lines about it in readme.

[orenergy]

Updated my request with your corrections. Please review