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.
There're some gramma mistakes in this code comment. I chanaged it as beblow, please verify it.
"It is the base class for handling block files. Block handlers for different under file systems can be implemented by extending this class. It also creates a specific BlockHandler for certain block file by checking the block file's path."