Added methods to CommonUtils

They come in handy across the code base
Fixed CommonUtils.concat check
More fixes to path manipulation functions
Reimplemented path manipulation function to use Apache FilenameUtils

We make sure all the FilenameUtil functions use the UNIX path separator,
which equals Constants.PATH_SEPARATOR
Updated documentation
Merge pull request #169 from manugoyal/common-util-additions

Added methods to CommonUtils