Fix issue 2555: Evaluate more terms for Laurent power series

http://code.google.com/p/sympy/issues/detail?id=2555

This doesn't fix only mentioned in [the comment 4](http://code.google.com/p/sympy/issues/detail?id=2555#c4) problem with "symbolic exponents".  I think, this one can't be solved in the current series framework.  More examples: [3136](http://code.google.com/p/sympy/issues/detail?id=3136), [3137](http://code.google.com/p/sympy/issues/detail?id=3137) and [4030](http://code.google.com/p/sympy/issues/detail?id=4030).