chainable constructor

This feature changes the constructor, as well as the ```read``` and ```merge``` methods to return ```this```, the Wkt.Wkt object itself.

This was mentioned in this issue, and its spirit is to have constructor, ```read``` and ```merge``` to have the same behavior that ```fromJson()``` and ```fromObject``` methods. It would also allow to chain methods i.e.

```js
new Wkt.Wkt().read("POLYGON ((30 10, 10 20, 20 40, 40 40, 30 10))").merge(new Wkt.Wkt('POLYGON((35 15,15 25,25 45,45 45,35 15))')).write();
```
returns 
```js
"MULTIPOLYGON(((30 10,10 20,20 40,40 40,30 10)),((35 15,15 25,25 45,45 45,35 15)))"
```