astrometry: return barcentric freq without Units (for now)
use python warnings module for warning messages
spindown: use correct TDB-based time for evaluating phase.
Add placeholder class for solar system shapiro delays.
Move all timing model stuff to models/ subdir.
Reorganizing all code into subdirs under pint/
test_model.py: update to import from new dir structure.
test_parfile.py: update to use new import syntax.
testtimes.py: update to use new import syntax
Update more tests to use new import syntax.
Merge branch 'scott'
Add basic working Sun Shapiro delay; still need to clean up code a bit.
Clean up SS shapiro code; add calcs for planets.
models: add StandardTimingModel with all basic components.
Put constants in top level PINT import
Added simple plotting and SS Shapiro delay to model