Implemented __call__ for UnitRegistry as replacement for []

getitem has been labeled as deprecated and we removed in 0.6.

Close #106
Removed duplicated units/aliases
Raise exception or warn upon redefinition of units.

In the DEFAULT_REGISTRY, an exception (ValueError) is raised.
In any other registry, issue a warning

The behaviour can be chaged using the `on_redefinition` argument in
the constructor.

Close #108
Update nonmultiplicative units docs.
Added docs about serialization.

Closes #109