add block handler to handle block files on different under file systems
add BlockHandlerHdfs as stub
Trying to fix HBase support issue in TACHYON-27
change isHadoopUnderFS() in UnderFileSystem to be public, and use it in BlockHandler
modify comment format and gramma mistakes
some format fix
Optimize imports; reformat code style; improve javadoc; remove the commented code.
Linked to TACHYON-46 in Javadoc
Improve Javadoc comments
Merge branch 'master' of https://github.com/amplab/tachyon into createNonRecursive
createNonRecursive() should throw exception if parent directory doesn't already exist,  which is extremely important for HBase to do fencing of dead region servers properly.
modify according to the comments, format change
fix typo
change error message
clean coode based on comments
Fix the bug mentioned in [TACHYON-66]

Fix the bug mentioned in [TACHYON-66] "The memory usage statistics is
not right when multi-user cache the same file and stops their
connection".
add test case for BlockHandlerLocalFS
Change the synchronized range
small format change
new example