Sped up unescape_new_line

I was seeing a heavy performance hit when getting larger objects through py4j (maybe the same problem discussed in issue #146). 
I traced the problem down to the unescape_new_line in protocol.py, which runs for a very long time on long strings. I have replaced the implementation with an implementation utilizing the native implementations of the python methods.

A comparison of performance: 
```
In [23]: print(time.time()); print(hash(unescape_new_line(about_a_million_character_string))); print(time.time())
1413979817.67
9136211936225273955
1413980082.09

In [28]: print(time.time()); print(hash(unescape_new_line2(about_a_million_character_string))); print(time.time())
1413980303.31
9136211936225273955
1413980303.32
```
So time taken went from from 264 seconds to 0.01 seconds.

I hope that you will merge this pull request.