This change is primarily concerned with documenting ExtensionManager,

fleshing out some details of some of the methods, in preparation for

some new work.
Some of the code has been cleaned up a bit just to better conform with

coding standards. Double quotes have been turned into single quotes,

line lengths are more consistent, and some dictionary key checks have

been turned into try/except blocks. The logic changes are minimal.

Unit tests pass.
Checked to make sure all common extension operations (loading, enabling,

disabling, configuring) worked.
Checked that docs built without errors.