Multiple changes

  - Added serial_echo to listen for integers over the
    serial monitor
  - Added serial_pwm_control to adjust the speed of a motor
    via serial commands on the serial monitor
  - Added a graph of V vs PWM input in the serial_pwm_control
    folder to determine the gain for the L293 chip when
    connected to 12V external power.
  - Added time_vs_theta to create time and position data on
    a motor in order to experimentally determine its transfer
    function.  There are also MATLAB scripts in the folder
    I'm working on to process the data
completed m file to solve for transfer function

  - found a and k for a simplified model of the encoded
    pololu motor.  The time constant was variable depending
    on the voltage applied...which frustrated the crap out
    of me.  So I am going to use the mean time constant and
    K value in the operating band for the motor.