the '_addTile' method now also handles tiles comming from a websocket.
Tiles are no longer added in one batch as they are added when received.
Added twip to pixel and pixel to twip convertor methods.
1 twip = 15 px
'onopen' and 'onmessage' events are handled
in src/layer/tile/TileLayer.js
Tile data is received as an ArrayBuffer which contains a string message
followed by a new line and then by the actual PNG data. The PNG data
is encoded in base-64 in order to be displayed.
