Add Font class, used to draw text from bitmap font

- The Font class takes an image as the "font sheet". This sheet consists
  of a grid of characters, starting from any ASCII index and ending at
  the end of the sheet - for example, your font sheet could include all
  the characters from ' ' to 'Z', or every single ASCII character from
  index 0 to index 255.
  - The Font class has the following public methods for drawing text:
    - glDrawText() : draws the given String to the screen.
    - glDrawTextLineWrapped() : draws the given String to the screen,
      wrapped within bounds at the character level. Any overflowing text
      is clipped.
    - glDrawTextWordWrapped() : draws the given String to the screen,
      wrapped within bounds at the word level. Any overflowing text is
      clipped.

- Enabled OpenGL blending (enables transparency).

- Changed x and y projection co-ords so that the top left is now (0,0)
  instead of the bottom left. Entities updated appropriately.

- Added methods in the main class to draw sample text.