world.js: Refactor move commands out of keyDown

In order to better handle move commands, refactor them out of the keyDown
event handler. This will enable using other input methods to trigger a move
command, such as mouse clicks or touch events.
world.js: Trigger movement with clicks on the map

Clicking on the map in different quadrants will move you in that direction.
Clicks are measured relative to the centre of the current location centre of
the map. Click above the player to move up, click below to move down etc.

This should enable better gameplay on touch devices.
Merge pull request #7 from Cogito/moveInterface

Improve movement interface to allow click events