Labeled all integration tests.

New unit tests are required to get full coverage. The tutorial should
also be updated.
Added unit tests to cover the doorstop.core.tree.
Full coverage with unit tests.
Added tree publishing to the tutorial.
'make doorstop' requires 'env'.
Merge pull request #46 from jacebrowning/integration-test-refactoring

Refactor tests to clearly distinguish between unit and integration.
Merge from 'master' branch.