Allow setting and resetting the global adaptation manager.

The global adaptation manager is used by traits for automatic
adaptation. Setting and resetting it is useful at application
start-up, but especially during testing to reset the global
state.
Fix traits tests that were registering adapters to the global manager.
Merge remote-tracking branch 'origin/master' into fix/global-adaptation-manager
Update CHANGES.txt
Document changes.
Fix typo in docs.
Fix typos.
Fix style issues.
Merge pull request #145 from enthought/fix/global-adaptation-manager

Allow setting and resetting the global adaptation manager.