TACHYON-237: Integrate Tachyon with secured HDFS.
TACHYON-237: Fix unnecessary import.
Merge remote branch 'upstream/master'
TACHYON-237. update for comments
Merge pull request #545 from shimingfei/AddDocForHierarchystore

modify and add more description
tweak doc for hstore
update beginner task's url
Merge pull request #541 from hitliuyi/master

TACHYON-237: Integrate Tachyon with secured HDFS.
format hdfs security code
remove unused imports
catch InvalidPath for getFileStatus to avoid infi loops
Merge pull request #547 from dcapwell/masterclientinvalidpath

catch InvalidPath for getFileStatus to avoid infi loops
update the description for storage quota configuration