Added RC-PWM based motor module with polymorphic motors

Was unsure about what to name the different motor types, went for
PWMMotor and TLEMotor although this is flexible.
Added TLEMotors to RASDemo

Tested with both types of motors
Renamed PWMMotors to ServoMotors

ServoMotors as a name helps differentiate between RC PWM based motors
and TLE motors which also use a sort of PWM to drive.
Merge pull request #36 from ut-ras/servomotors

Adding Servo Motors