Allow `datetime.date` values with `columns.DateTime`.

Adds support for setting a `datetime.date` value to a `DateTime`
column. The value will be transparently converted to a
`datetime.datetime` with the time components set to zero.
Log the CQL statements at DEBUG level.

This logs the CQL statements emitted by cqlengine in the
`cqlengine.cql` logger at DEBUG level. For debugging purposes you can
enable that logger to help work out problems with your queries.

The logger is not enabled by default by `cqlengine`.
Merge branch 'logging' of github.com:dokai/cqlengine into pull-50

Conflicts:
	cqlengine/columns.py