add block handler to handle block files on different under file systems[Tachyon-33][SubTask 1]
modify according to comments
make close() simpler
add some negative test case
use guava's closer to replace current code path
rename test cases for BlockHandlerLocal
Merge pull request #395 from shimingfei/AddBlockHandler-new

add block handler to handle block files on different under file systems[Tachyon-33][SubTask 1]
remove license temporary
added exponential backoff for master client
docs