Added the V2 public marketdata method and changed URI. Added singlemarketdata and singleorderdata methods.

A small extension of the existing class. I also added a array where you can store all the public methods which can be called. I think this way it's a little easier to add new methods in the future.

```Javascript
publicMethods = ['marketdata','marketdatav2','orderdata','singleorderdata','singlemarketdata'];  
```