Added several classes for handling mathematical expressions.
Changed expressions to return floats rather than ints, which will make
them a bit more useful. They already did their math with floating point
precision, just now they don't truncate the result.
Minor update to VariableExpression so it doesn't get messed up if it
can't find a variable.
Bugfixes for expressions.
Gave VariableExpression an accessor for its variables.