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.
test_model: update to use StandardTimingModel function.
Put constants in top level PINT import
Added simple plotting and SS Shapiro delay to model
Updated required packages.  Fixed typo.
Another typo fix
Fixed conflicts