Updating doxygen documentation.

Added mechanism to document API changes in the generated doxygen
documentation. Developers are now expected to keep MajorAPIChanges.md
file updated as they change APIs or major design features.

This change now requires doxygen 1.8 or greater since we now rely on
markdown support in doxygen.

Change-Id: I55b340625cbd21cff7913d1607062c0cb4d72465
Merge topic 'cleanup_doxygen'

404d9b79 Updating doxygen documentation.