allow to define groups of markers.

Hey there!

I've worked a little with your project. Works great and I like it :D
Though, I was missing to from groups of markers, allowing different colouring for each group.

**Here is my proposed solution**:

- It allows to specify groups,
- and each group can have their own marker colour and size (I'm not sure if I've missed some other settings). 
- If a group does not have set specific settings, the default ones will be taken.

The other methods to add markers still seem to work as before (as one can see when opening the 'index.html').

I've also tried to update the docs as good as possible, but you might want to double check on that :)
![marker-groups](https://cloud.githubusercontent.com/assets/1180239/3351294/f423c9bc-f9ff-11e3-8c8c-fd0599f91860.png)

 (Bear in mind: I'm not a JavaScript developer, so I'm not entirely sure if I've used some nasty hacks.)