expand range of pins servos can be attached to

Addresses this issue: https://github.com/firmata/arduino/issues/62.

Changes the way servos are mapped to pins in order to enable servos to be used on any supported digital and analog pin. For example, with this update you can attach a servo to an Arduino Uno on digital pins 2-13 or analog pins A0-A5. In the past you could only attach servos to digital pins. On an Arduino mega you can now attach a servo to digital pins 2-53 or analog pins A0-A9 (there appears to be is a limitation in the Servo library that prevents attaching a Servo to a pin number higher than A9/D63). If the maximum number of servos is exceeded for the board, the string "Max servos attached" is be returned via Firmata.sendString().

@ntruchsess we'll want to add this to configurable as well.