Improved _createFile method

The current method was doing a lot of redundant work by traversing the
tree multiple times. Now we use the traverseToInode method to get as far
down into the tree as possible and create the rest of the files from
there.
Fixes

Fixed indentation and removed unnecessary stuff. Replaced InodeTraversal
class with a generic pair, and added a test for the listFiles fix.
Added comments
Merge pull request #180 from manugoyal/master-info-create-file

Improved _createFile method
Merge pull request #181 from Du-Li/master

more tests on UnderfsUtil.loadUnderFs and bug fixing
fix tachyon.command.Utils.validatePath: use CommonUtils.concat, API documentation
add unit tests for tachyon.command.Utils
Merge branch 'debug-loadufs' into pr186