Fixed TACHYON-97
Several minor changes as dcapwell  mentioned
Minor changes as haoyuan mentioned
add storage dir to manage and store block files on different under file systems
some format change
add eviction strategy for swapping block files
some format change
update BlockHandler, change final variables and remove lastTier from interface
delete ufsConf when getting BlockHandler
modify according to comments
modify according to comments, add more synchronization control
add test cases for StorageDir
replace  dirIndex using StorageDir
make eviction stragey interface to accept the storage dirs candidate as input
modify code style
rename tachyon.worker.elimination to tachyon.worker.eviction
modify according to comments, use ConcurrentHashMap to replace HashMap
Integrate TestDFSIO unit test; Enhance boundary condition checks as well as unit tests for seek() methods in input streams.
clean up TestDFSIO  code
Remove commented lines in pom.xml