Added datetime cell comparison.

Adds cell <=> datetime comparison which follows the rules of cell comparison on points/bounds and the implementation of each datetime's comparison operator on single values. 

Result - you can run the following code:
```
import iris
filename = iris.sample_data_path('uk_hires.pp')
cube = iris.load(filename)[0]
with iris.FUTURE.context(cell_datetime_objects=True):
    print cube.extract(iris.Constraint(time=iris.time.PartialDateTime(hour=11))).coord('time')
```