Fix ordering select widget

[rpkilby]

Fixes #496. Supersedes and closes #499.

• Changed CSVWidget => BaseCSVWidget
  • The rendering behavior only forces a text input when there are multiple values to render (ie, you directly modify the url query params to have multiple values).
  • If there are one or zero values, the mixed-in widget is used to render (such as select, date input, etc...), making the widget suitable to use in forms.
  • Form usage will generally only for allow one input (e.g, a select/date input can only provide one value).
  • API usage allows for comma-separated values.
• Re-added CSVWidget class that is the base widget pre-mixed w/ TextInput. This mimics the previous behavior and is used by the CSVRangeField, as range only accept two values via comma separation.
• BaseCSVField will now mix the main widget class w/ BaseCSVWidget (making CSVSelects, CSVDateInputs, etc...). This the same way a mixed-in csv filter will make it's derivative mixed-in csv field.

[carltongibson]

This is really rather good 🙂.