Split up doorstop.core.base into base and types.
Added a placeholder Level class.
Cleanup.
Implemented the Level class.

Still not used anywhere.
Refactored the Level into item.py
Refactored Level into document.py
Verbified a function.
Merge from 'master' branch.
Merge pull request #50 from jacebrowning/level-class

Refactor level tuples into a Level class.