Default construct rep inside quantity

[chiphogg]

This won't change any actual values when the rep is a fundamental
arithmetic type, but it is more stylistically consistent with the idea
of a "default constructor".

We also update our contract to officially promise that we do this. This
lets us get rid of the scary/threatening language that "we might change
this out from under you!", even though we never had any intention to
actually do so. We retain the encouragement to use au::ZERO, and to
avoid using a default-constructed Quantity for anything other than
future assignment.

Fixes #273.

[geoffviola]

Makes sense. This feature is a departure from initializing fundamental types in C++. But it seems like the right trade-off in terms of safety and clarity.