- Change gitignore so it doesn't ignore Mock files.
- Added NATRON_ENABLE_MOCK to instrument the code so it uses mocks
instead of real objects.
-  Added the first (not complete) definition of a mock: See
OfxEffectInstance_mock.
- We should now use these mocks in the unit tests.
Remove useless enum value APP_BACKGROUND_MOCK
as we use the NATRON_ENABLE_MOCK define instead.
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop