Issue 1966: Add evaluate=False to sympify

Changes:

- Add `evaluate=False` flag to `sympify` (fixes [issue 1966](https://code.google.com/p/sympy/issues/detail?id=1966))

I know that we don't want more flags, but this isn't a transformation on the tokens like implicit parsing is, so for now I've used the API as discussed in [issue 1966](https://code.google.com/p/sympy/issues/detail?id=1966).

This works by using an AST transformer to change `+`, `-`, `*`, `/`, and `**` operators into the SymPy equivalents and add `evaluate=False`.