Run code as a paragraph

This PR gives ability to run code as a paragraph, programmatically.
By implementing

```
ZeppelinContext.run(String code);
```

For example, if there're one paragraph like,
```
%md ### Hello Zeppelin
```
then, equivalent code is 
```
z.run("%md ### Hello Zeppelin")
```

This feature is useful because it can construct paragraph programmatically. For example

![image](https://cloud.githubusercontent.com/assets/1540981/4775150/f46711ba-5bb4-11e4-8ece-5af8980b236d.png)