Pad the operands in evalBinOp to work w/ops that lex like an identifier

When op = in, this runs eval('v1' + 'in' + 'v1') === eval('v1inv2')
A similar problem occurs when op = instanceof. We want to put a space
around the operands so that these cases are properly evaluated as an
expression.
Merge pull request #5 from btlachance/evalBinOp-operatorSpacing

Pad the operands in evalBinOp to work w/ops that lex like an identifier