Expose sprite map width and height

[agrobbin]

If a user doesn't provide an image name when using spritely-height and spritely-width, we now return the whole sprite maps' height and width, respectively.

For example, if a sprite map's outer width and height are 100px and 200px:

.test {
  width: spritely-width('test');
  height: spritely-height('test');
}

This CSS will be returned:

.test {
  width: 100px;
  height: 200px;
}

Closes #20.