Add column types

@gulopine

The real changes here follow 994d031 (the first two commits are just #1 and #3). This PR makes the following changes beyond those two:

1. Add `UnicodeColumn`: it's like StringColumn, but emits Unicode (by default, 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 datetimes. Extends previous functionality by also allowing multiple input formats, and the specification of a timezone into which parsed datetimes should be localized.

All new column types have tests added, too, all of which pass.

Also a handful of minor PEP257 fixes around docstrings.