Fix operator resolution case

An unresolved operator can be preceded by an operator of unknown arity, which means that the unresolved operator should most likely be a unary operator.
DDMathEvaluator.angleMeasurementMode.  Fixes issue #29.

DDMathEvaluator has a new property to specify that the arguments to the trigonometric functions should be interpreted as being in degrees versus radians (the default).
The logical functions needed to be able to handle an evaluation returning nil.  Fixes issue #43.
Exploring parsing strings as variables.

@"\"Foo\" + \"Bar\"" would be equivalent to @"$Foo + $Bar"
@"'Foo' + 'Bar'" would be equivalent to @"$Foo + $Bar"
Numbers can now be entered as hex, beginning with either "0x" or "0X".

This does not interfere with the multiplication operator, because "x" is not recognized as multiplication; "*" and "×" (the actual unicode multiplication sign) are.
Fixing format string warning.