interrupt driven stepper controller inherited from AccelStepper

To use this module inherit from the AccelStepper class (http:https://www.airspayce.com/mikem/arduino/AccelStepper/), modify it to expose necessary members into public:

_stepInterval _direction _currentPos _targetPos _speed _pin[4]

Within Energia on LM4F target there are only two timers available for free use: Timer1 and Timer2

// SYSCTL_PERIPH_TIMER0, // wiring_analog.c analogWrite() // SYSCTL_PERIPH_TIMER1, SYSCTL_PERIPH_TIMER2, SYSCTL_PERIPH_TIMER3 // SYSCTL_PERIPH_TIMER4, // Tone.c // SYSCTL_PERIPH_TIMER5, // wiring.c - millis() micros() // SYSCTL_PERIPH_TIMER6, // SYSCTL_PERIPH_TIMER7