Refactor configuration class

Current configuration is based on system properties. This results in a long command line string,  hard to customize and change dynamically.

This pull adds a new configuration class using configuration file, the same way as Hadoop has with its core-site.xml. 

For compatibility, both system property and xml configurations are supported. The overwrite order is system > file. This is also inspired by what SPARK does (thanks to Haoyuan's pointer).
Please see the coding style guide: http://tachyon-project.org/master/Startup-Tasks-for-New-Contributors.html

It also has a Eclipse formatter.