Support for different PWM pins and the Mega 2560

Hello, I am using this library to drive motors on an "Antenna Tracker" project. We use the Arduino mega however and wanted to use the same accurate PWM. So I modified the library to allow for use of any PWM pins. It checks if a timer is compatible with the PWM pins selected and if so it uses the accurate PWM otherwise it just uses analogWrite. It should also be fully compatible with the previous versions of the library.

It is not fully test for all the pins on the mega 2560 but it works perfectly using Timer5 for the pins we broke out to the driver board (44 and 45 I believe). It should be exactly the same for the other timer pins so there is no reason it wouldn't work, just want to make it clear that it wasn't fully tested.

Hope you can use my additions!