A bit more encapsulation

take the model parameters from the `constants` module and
put add them to the calculator object. Now all parameters and
data live in a calculator.
`calculate` function makes a Calculator

taxcalc.calculator takes some data and kwargs
fix path on test file
Update README.md
Merge pull request #58 from talumbau/const_mod

Merging in preparation for more refactoring...
Update README.md
resolved merge conflicts
implemented one big output dataframe