early termination in diff

[jverzani]

This implements suggestion in #225 to have a step for early termination in diff if possible.
This passes on #225; the additional allocations to check for early termination would be paid in every instance; whereas the suggestion of #225 only pays off in certain situations. (The allocation could be avoided by some call to basic_has_symbol, but even that had a performance cost)

• extends diff interface closer to SymPy by allowing expressions like diff(ex, x, 2, y).
• does not cover diff(ex, (x,2)) interface of SymPy, as that method is used otherwise. (would fix were it not breaking)
• adds tests
• clean up one verbose test by using repr

[jverzani]

There are two things here: one extends the interface, the other closes #225. The latter adds a check for whether the expression is zero. As such, it may be a performance concern. I didn't benchmark, But if there is a concern, I can modify this PR.