refact getFileStatus in TachyonFs
Merge pull request #519 from shimingfei/TachyonFSfix

refact getFileStatus in TachyonFS
update documents
add test cases for AllocateStrategy
some modification on EvictionStrategyTest, delete Thead.sleep and unused Exceptions
Merge pull request #531 from shimingfei/AddTestCaseForAllocateStrategy

Add test case for AllocateStrategy and some modifications on EvictStrategyTest