Add basic caching of intermediate results in TimingModel.
Reorganize caching code a bit.
Merge branch 'master' into cache
timing_model: init caching properly
Merge branch 'master' into cache

Conflicts:
	pint/models/astrometry.py
Merge branch 'master' into cache
Merge branch 'master' into cache
Merge branch 'master' into cache

Conflicts:
	pint/models/astrometry.py
	pint/models/timing_model.py
Updated test for newest TDB bug fix
Merge branch 'paul'
spindown: use TDB for pepoch
Add origin/object labels to PosVel vectors.
Merge branch 'paul'
Added pickling of TOAs.  Big speed-up when re-reading.
Changed string-based angle unit definition
Removed lat/lon resets.  Added TOA .TDB param which is effectively a cached value