Clean up legacy examples

[mdickinson]

[Includes commits from #556]

This PR cleans up our legacy examples, either deleting them or making them work:

• Makes the AcmeLab workbench-based example work again, primarily by fixing Python 2 relative imports.
• Removes two egg-based demos: AcmeLabUsingEggs and MOTD_Using_Eggs.
• Removes the Lorenz demo, which uses parts of Chaco that no longer exist (ChacoPlotItem)
• Fixes flake8 issues (mostly unused imports and missing copyright headers) in remaining examples
• Removes the examples directory exclusion from the .flake8 configuration file

Closes #381 Closes #361 Closes #329

[mdickinson]

Updated to remove an outdated doc page related to MOTD_UsingEggs (that Sphinx was already complaining about because it wasn't reachable).