add block handler to handle block files on different under file systems[Tachyon-33][SubTask 1]

details are on Tachyon's JIRA
BlockHandler is used to read and write block files on different under file systems, currently local file system is realized.
I assume most BlockHandler concrete classes need to implement this? I f that the case could we throw UnsupportedOperationException instead. This would in inline with like Hadoop FileSystem class.