(PUP-511) Remove Non- and Q-Values

The language is getting fixed up so that there isn't any need for
Non-Value or Q-Value expressions. Q-Values are handled by just
specifying the precedence rules and depending on that. Non-Values are
going away by instead defining a value for every expression.
(maint) Correct word for "commutative"

The property in which changing the position of the operands does not
change the result of the operation is called the "commutative property".
(PUP-511) Remove references to Q-Value

This removes a few references to Q-Values, which are going away. It also
has some small style cleanups.
(PUP-511) Remove Non-Value expressions

This removes the final references to non-value producing expressions.
(PUP-511) Specify resource expressions

This brings the specification of resource expressions in line with the
work done for PUP-2898 as well as clarifying some of the semantic
constraints on the expressions (no duplicate titles, no duplicate
attributes, etc.)
(maint) Add information for Default type

The default expression was given the type Default, but we forgot to add
it to the language specification.
Merge pull request #14 from zaphod42/update/master/pup-511-specify-resource-expressions

(PUP-511) specify resource expressions