Added warnings for duplicate Item levels in a Document.
Merge from 'master' branch.
Fixed duplicate test.
Docstring cleanup.
Added warnings for skipped Item levels in a Document.

Still need to check the following:

- 1.1 —> 3.0
- 1.1.1 —> 2.1
- etc.
Refactored to make testing easier.
Added unit tests for level checks.
Implemented all document level warnings.
Added over and out level checks.

Fixed #28
Fixed a test.
Merge pull request #34 from jacebrowning/level-validation

Add warnings for duplicate and missing Item levels in a Document.