Remove the ImportError try/except

There is no reason to catch this error --- if it happens, we should let it
propagate.
Let sympy2csympy() handle the exception raising
Convert lists and tuples in sympify()

Add tests.
Simplify Mul::as_base_exp()

Essentially there are only two special cases, and the rest is just the general
case handled by the "else" statement.
Merge pull request #296 from certik/sympify

Improve Sympify