### What is this PR for?
Output from interpreter is displayed after completion of paragraph execution.
It'll be useful if output can be streamed to front-end during execution.

Previous work #593 injects InterpreterOutput stream object to Interpreter.
This PR is based on #593 and stream the data from InterpreterOutput to front-end.

This implementation only streams output is %text. Other output type (%html, %angular, %table) is not streamed to the front end.

While this PR keeps backward compatibility, Interpreter who want to use this feature will need to modify code to write output into `InterpreterOutput` instead of return with `InterpreterResult`.

This PR includes modification of SparkInterpreter to use InterpreterOutput.

### What type of PR is it?
Feature

### Todos

### Is there a relevant Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-554

### How should this be tested?
Run such code using Spark interpreter
```
(1 to 10).foreach{ i=>
    Thread.sleep(1000)
    println("Hello " + i)
}
```

### Screenshots (if appropriate)
![stream_output](https://cloud.githubusercontent.com/assets/1540981/12188677/6df08900-b56c-11e5-95d9-e5f6fad91007.gif)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Lee moon soo <[email protected]>

Closes #611 from Leemoonsoo/output_stream_frontend and squashes the following commits:

53e2bb4 [Lee moon soo] Not persist on every append
dedae0d [Lee moon soo] Remove debug lines
8251fb4 [Lee moon soo] Fix syntax and style
9c9c8fd [Lee moon soo] update test
18215a3 [Lee moon soo] fix style
f7e6a4d [Lee moon soo] Fix syntax error
d29cfbf [Lee moon soo] workaround jshint
07b3e1a [Lee moon soo] Handle clear output correctly
bc6262e [Lee moon soo] Make PysparkInterpreter stream output
6d9cc51 [Lee moon soo] Pass InterpreterOutput to SparkILoop
b68180e [Lee moon soo] Add InterpreterOutput on spark interpreter unitest
626ad48 [Lee moon soo] Add license header
846015b [Lee moon soo] Update scalding
37d6920 [Lee moon soo] Handle display system directive correctly
e278e84 [Lee moon soo] Clear output correctly
479b836 [Lee moon soo] Add test
c01df62 [Lee moon soo] Connect Spark interpreter Console.out to outputstream
8a1223f [Lee moon soo] Handle update output correctly
e7a9b37 [Lee moon soo] Delayed persist
2060c1e [Lee moon soo] Clear before render text
786c978 [Lee moon soo] update paragraph object after witing to outputstream
258ff38 [Lee moon soo] Barely working
6f607f7 [Lee moon soo] Add newline listener
89d9798 [Lee moon soo] Render text output line by line
a42e4ff [Lee moon soo] Update test
fb5e7b5 [Lee moon soo] Update test
0f60b54 [Lee moon soo] Update test
a07d7db [Lee moon soo] Implement InterpreterResult.toString
1f419b6 [Lee moon soo] Add InterpreterOutput
c91f498 [Lee moon soo] prepend interpreteroutputstream to interpreter result