added datasource as variable type

With this PR now it is posible use variable to change datasources in query variables and graphs

![image](https://cloud.githubusercontent.com/assets/5883405/5023638/f1cf7d96-6afc-11e4-8c1c-aa2fcf0015a8.png)

Solves (https://github.com/grafana/grafana/issues/816)

![image](https://cloud.githubusercontent.com/assets/5883405/5023650/1804f018-6afd-11e4-8d9d-dd02d2f7f9b3.png)

![image](https://cloud.githubusercontent.com/assets/5883405/5023676/79dc3e04-6afd-11e4-9fd1-0a5997be001e.png)