Basic TimeConstraint(hour=...) capability

This is a minimal-capability, first-draft implementation of a `TimeConstraint` class which targets a [discussion group posting](https://groups.google.com/forum/#!msg/scitools-iris/qD5bAvs7OK8/oPt2h4lUxRwJ). As such it only supports selecting based on the hour, and only inspects coordinate point values. The API is intended to be open enough to allow easy extension to year, month, day-of-month, minute, etc.