Merge in 1.6.x

Of the conflicts resolved, these don't appear in the files-changed (cos' the results just revert version changes in the 1.6.x history)
 * ```lib/iris/__init__.py```
 * ```setup.py```

That just leaves ```lib/iris/tests/unit/cube/test_CubeList.py```, which had test classes added in both parents.  Combined includes and kept both tests. 

**update** later reworked to resolve merge conflicts arising when master changed (cf. https://github.com/SciTools/iris/pull/1082#issuecomment-38904286) :
```
Conflicts:
	lib/iris/analysis/maths.py
	lib/iris/tests/unit/analysis/maths/__init__.py
	lib/iris/tests/unit/analysis/maths/test_add.py
	lib/iris/tests/unit/analysis/maths/test_divide.py
	lib/iris/tests/unit/analysis/maths/test_multiply.py
	lib/iris/tests/unit/analysis/maths/test_subtract.py
```