Made some improvements

Major Fixes:
I fixed an issue when receiving extended IDs larger than 0x7FFF.
I fixed the freezing issue with the receive example when its used on a utilized bus.

Minor Fixes:
Changed receive example to include the ID and I changed how the output is formatted.
Replaced some division arithmetic in mcp_can.cpp.
Changed some bad baud-rate settings, then tested the ones I could.
Updated the README.md file to contain some useful information and to clarify that this library is not only compatible with the CAN_BUS_Shield but any shield or CAN interface using the MCP2515 with an Arduino.