Doc-Doctesting

Initial commit to test building on travis. Inculdes some initial fixes to the .rst files which were added while debugging the errors on my machine. 

Extension of issue #588 
Suggest instead 
```Python
    >>> by(iris.species,                # Group by species
    ...    min=iris.petal_width.min(),     # Minimum of the petal_width column per group
    ...    max=iris.petal_width.max()),    # Maximum of the petal_width column per group
```