Make Quantity and Time/TimeDelta consistent attribute naming

As @taldcroft  noted in #751, the `Time` object and `Quantity` objects have some similar-sounding attributes that are not quite the same.  `val` and `vals` for `Time`/`value` for `Quantity`, and `is_scalar` for `Time`/`isscalar` for `Quantity`.  We should probably adjust these to be the same.  Then people don't have to try to remember which is which.

Personally, I'm for `value`/`values` over `val`/`vals` - it's only a couple more characters and quite a bit more readable. 

`isscalar` vs `is_scalar`:  I think I wrote `isscalar`, and I think I did it that way because it's how the `numpy` function is named.  Also, it's a property rather than a method and attributes/properties in my head aren't underscore-delimited.  But I don't think that's in PEP8, so I have no strong opinion.

I'm tagging this for 0.3, but if this quickly settles on an answer and is easy to implement, we might slip it into 0.2 to keep from breaking compatibility (@iguananaut ?)