Fix for issue 1756
Implement _eval_as_leading_term for UndefinedFunction, second part of issue 1756
A correction for Pow._eval_nseries: case 1/f(x)
Don't divide to order term while evaluation of series 1/f(x)**e
Add correct Order term in Mul._eval_nseries()
Removed redundant Symbol's declarations in series/tests/
Fixed several tests after modifications in Pow and Mul
Fix test_loggamma:

    It seems that answer was wrong: x**2*log(x)/2 is missing.  See
    also wolframalpha series:
    http://www.wolframalpha.com/input/?i=LogGamma[1%2F%28x+%2B+Sin[x]%29+%2B+Cos[x]]
Fix issue 3683: don't fail in Expr.series if s1.getn() != n
Add tests for issue 3554
Fix for issue 3111