fix for issue 3229 - evalf fails on some sums

- Corrects a bug in limit evaluation of Sum.euler_maclaurin()
- add tests for evalf_sum
Catch symbolic relational error in Sum.evalf().

Partly fixes issues 3229. The remaining part has to do
with the .evalf() call when printing. This happens
because Sum(f(x), (x, 1, 3)).is_number is True which,
according to current definitions, seems correct.

http://code.google.com/p/sympy/issues/detail?id=3229
bug fix, more pep8 compliant
comment for fast Sum test
Return early if we can't evalf the summand.
revised symbolic/evalf logic in Sum.euler_maclaurin()