Support lazy operations in cube.collapsed.

Derived from the more experimental https://github.com/pp-mo/iris/pull/5

**Will not work at present**
Because biggus still needs a couple of things...
  - [x] a variance aggregator : https://github.com/SciTools/biggus/pull/57
  - [x] a working ```biggus.Aggregation.masked_array()``` method