Made some improvements

Major Fixes:
Fixed an issue when receiving extended IDs larger than 0x7FFF.
Fixed the freezing issue with 'receive' example when it is tested on a utilized test bus.
Changed some incorrect baud-rate register constants, and tested some against a Peak-System PCAN-USB dongle.

Minor Fixes:
Changed 'receive' example to include the ID and changed how the serial output is formatted.
Replaced some division arithmetic in 'mcp_can.cpp.'
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.