More accurate validation for date & time in studio editor.

TNL-58
Added email address to AUTHORS.
Using ISO_8601 for date formatting instead of string.

TNL-58
Comment refactoring.

TNL-58
Adding more readable comments to catch block.

TNL-58
Merge pull request #638 from edx/muzaffar/tnl58-date-validations

More accurate validation for date & time in studio editor.