Config refactor - add support for KVP + XML configs

This pull adds support for a KVP tachyon.properties and tachyon-site.xml configuration.

Both files are supplied in the JAR and loaded from the job for default values.  Either can be overridden with class loader loading priority.

The weight of properties is:
System/ENV > tachyon.properties > tachyon-site.xml

Properties should move out of code and into the tachyon.properties file (next refactor).  Also all System.setProperty(..) and System.getProperty(...) calls should be replaced/removed in favor of a shared config object.

The new config specifies ${USER.HOME}/.tachyon as the tachyon home directory.  This will assist in multi-tenancy and security concerns later.
there's two good options for single config:
1) apache common config.  supports multiple input formats with custom loaders, source weighting, etc.
2) hadoop config class - supports XML + env support with env weight.

Option #2 works great except no KVP support.  Option #1 works well too but resulted in more LOC for all the requirements to work (System property + KVP+XML).  #2 is already used in the UnderFileSystem classes, so I used that here too.  It was lacking KVP support so I added that as well.  This pull is the least amount of code for all the features.