interrupt driven stepper controller inherited from AccelStepper

To use this module inherit from the AccelStepper class (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