resolve UnderFS hostname in loadufs, add methods in tachyon.util.CommonUtils and unit tests
resolve UnderFS hostname before using in loadufs
some minor code style fixes
fix test cases to cover empty and null, and resolveHostName
fix resolveHostNameTest
Merge pull request #188 from Du-Li/resolvehostname

resolve hostname in UnderFS address in loadufs, add unit tests and documentation