Add ArrayOrNone trait type.
Allow override of the comparison mode.
Allow non-None default for ArrayOrNone.
Style nits: rename a private method, adjust spacing, add comments.
Add tests for non-sharing and validation of the default value.
Add docs.
Replace an old XXX comment with a comment explaining the intent of the code.
Fix style of new code to match that of existing code.
Remove unused imports.
Merge pull request #219 from enthought/feature/array-or-none

Add new "ArrayOrNone" Trait type.