Add new column types

@gulopine

The real changes here are in b891848. This PR makes the following changes beyond #1:

1. Add `UnicodeColumn`: it's like `StringColumn`, but emits UTF-8 when asked to output.
2. Add `FloatWithCommaSeparatorsColumn`: like `FloatColumn`, but handles data in which there are comma thousands-separators inside numbers
3. Adds `BooleanColumn`: mapping for columns holding true/false data
4. Adds `DateTimeColumn`: a generalization of `DateColumn` that allows general `datetime`s. Extends previous functionality by also allowing multiple input formats, and the specification of a timezone into which parsed `datetime`s should be localized.

Also a handful of minor PEP257 fixes around docstrings.