Add basket elements write APIs

I've added the `POST`, `PUT` and `DELETE` write API methods for the basket elements.

Here they are:

![capture decran 2014-03-20 a 14 11 49](https://f.cloud.github.com/assets/103900/2472121/755f10c2-b031-11e3-9a70-a8a0b31c00e7.png)
![capture decran 2014-03-20 a 14 11 55](https://f.cloud.github.com/assets/103900/2472126/7d8430e8-b031-11e3-9105-d03c18a7a2c0.png)