Issues with fixed timestep in Emitter and Spritemap; issues with usePositions in Grid

Fixed frame rate issues discussed here:

  http://forum.haxepunk.com/index.php?topic=603.0

Emitter and Spritemap should both be using assignedFrameRate to determine elapsed time when using a fixed timestep. Also Spritemap's documentation is wrong, frame rate is always supplied in frames per second.

As for the Grid issue when using usePositions, setRect does not work because it accounts for usePositions twice, resulting in incorrect locations. Improved saveToString so you can specify your own characters for rendering a string version of the grid. Clarified some Grid documentation.