Added more details about the problem statement (#1367)

* Update DIRECTORY

* Updated DIRECTORY

* Fixed bug in directory build and re-build the directory.md

* fixed url issue

* fixed indentation in Directory.md

* Add problem-18 of project-euler

* Delete sol1.py

* Delete files

* Added more details to question

* Added doctest in printNGE()

* Made changes to fix Travis CI build

* Remove the trailing whitespace