Enabled make doc target (Doxygen).
Merge pull request #288 from est77/enable_doxygen

Enabled make doc target (Doxygen)
Update README.md
rendermanager.py 2.4: print ping times.

also increased pause between updates from 30 to 60 seconds.
when building triangle trees, ignore moving triangles with an empty bounding box.
rendernode.py 1.18: improved error handling.
Merge branch 'master' of github.com:appleseedhq/appleseed
fixed formatting.
updated Doxygen input file and added appleseed logo.