Interactivity

this branch adds some methods to get data from the torque raster though the method getValueForPos. It allows to do interactive maps as shown in leaflet_interaction.html example.

Also I've updated the documentation (it was a bit outdated) and added the new methods
Shouldn't be this?
```js
xx >= 0 && yy >= 0 && xx <= 255 && yy <= 255
```
or
```js
xx >= 0 && yy >= 0 && xx < 256 && yy < 256
```