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.
@haoyuan 
Hi haoyuan, I think BlockHandler only offers some basic read/write method for handling block files, how to / when actually write to the file is determined by the caller(whether to store into a buffer first and write into the file when closing it)