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.
@dcapwell 
I think BlockHandler only offers some basic interface to process block files. 
The caller needs to store the data into some buffer first, when the buffer is full or the file is to be closed, call this function to write the data.
what is your idea?