Evaluate variables about aforge HOT 2 OPEN

I decided to take a stab at this and issue 4. In the process, I basically 
replaced the entirety of Core/PolishExpression.cs. For this reason, I do not 
recommend trying to read its diff; just treat it as "entirely new file with 
only minimal relation to to the old one".

Be aware that the implementation I chose for variable substitution will 
tolerate being passed fewer variables than are referenced, and ->will 
renumber<- the out-of-range variables: If the variables { 1 2 } are substituted 
into "$0 $1 $2 + +", the result is "1 2 $0 + +".

I also added tests; they should be pretty self-explanatory.