New DataFrameSchema API

Release Notes

• Major change: This release updates changes the API of the DataFrameSchema object.
  Instead of passing a list of Columns, you now pass a dictionary where the keys are column_names
  values are Column objects. This makes the API feel a lot more familiar for pandas users, who may
  often define DataFrames in a similar way (see README for details).
• renamed Validator to Check for brevity and clarity (accordingly renamed validator_{input, output}
  to check_{input, output}.
• created convenience variables for PandasDtype so they can be accessed in pandera namespace:
  Bool, DatetTime, Category, Float, Int, Object, String, Timedelta