Added absolute path expansion of parent directory check in saver.py, … (#6601)

* Added absolute path expansion of parent directory check in saver.py, so as to correctly handle saving files in current directory

* Added testSaveToCurrentDirectory(). This should fail on Windows according to https://github.com/tensorflow/tensorflow/pull/3695#r94327053 (not verified yet)

* Added fix to problem with saving graph to current directory explained in https://github.com/tensorflow/tensorflow/pull/3695#r94327053

* Removed all changes, bringing code back to state from 55b01593

* Added testSaveToCurrentDirectory() that uses filename without ./ in front of it. Test fails on restoring graph

* Added a fix to Saver.restore() to correctly handle restoring graphs from current directory when no "./" is prepended to path

* Added fix to issue explained in https://github.com/tensorflow/tensorflow/pull/3695#r94327053 with graph saving failing when path is a file in current directory with no "./" prepended.