PARQUET-75: Fixed string decode performance issue

Switch to using 'UTF8.decode' as opposed to 'new String'

https://issues.apache.org/jira/browse/PARQUET-75

Author: Daniel Weeks <dweeks@netflix.com>

Closes #40 from dcw-netflix/string-decode and squashes the following commits:

2cf53e7 [Daniel Weeks] Fixed string decode performance issue