bake: Fix bugs introduced by bake optimizations, of course. - Make the execution stats JSON-serializable. - Re-add ability to differentiate between sources used during segment rendering and during layout rendering. Fixes problems with cache invalidation of pages that use other sources. - Make taxonomy-related stuff JSON-serializable.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1