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.
merging in remote changes
Adding more NSDecimal functions
Updated project settings.
Converting to modern literal syntax
Clean up the implementation of random().  Fixes #34.
Implement some more high-precision trig functions
What could be more worthwhile than rewriting comments?
Use new high-precision functions
Function resolution should pass variables through
Modernize ARCHS setting
Exploring parsing strings as variables.

@"\"Foo\" + \"Bar\"" would be equivalent to @"$Foo + $Bar"
Pruning unused methods from the tokenizer
Reparenting DDMathStringTokenizer
Renaming methods to conform to what NSEnumerator declares
Moving ivars to inside @implementation blocks
Modernizing some syntax