[evaluator] Fix wrong evaluation of powers involving unary minus sign in base

https://code.google.com/p/speedcrunch/issues/detail?id=160

This workaround fixes the issue by:
    . prepending a zero to the expression in case it starts with a unary minus
    . replacing occurences of (- with (0-
This way, the minus sign is not unary and the power operation is correctly
evaluated by the current parser. A proper fix in the future is to rewrite the
parser.