Bug Fix for showNumber Negative Numbers

v1.5.2

• Fix showNumber() but for negative numbers. Solves issues #17 (thanks to @otpfiste).
• Fix showNumber() overflow condition for numbers (positive and negative) that do not fit within display length.

  // Example of negative case that did not render correctly
  display.showNumber(-3.1, 1, 3, 1);  // (float num, decimal length, length, position)

  // Overflow Examples - will render a dash display e.g. "----" 
  display.showNumber(-1000);
  display.showNumber(10000000);
  display.showNumber(-333.1, 1, 3, 1);