Vector-valued evaluation

Looking at #5, this is one of very few things left to do before we might call ourselves ready for a version 0.1.

I still think this makes sense only in a multi-linear context; the following is probably a good method definition for that:

``` julia
getindex{T,N}(itp::Interpolation{T,N}, NTuple{N,AbstractVector}...)
```

Of course, it's going to have to be more specific in order to avoid ambiguity errors. I'm thinking we might not have to specify the type of the vector elements at all - we'll get method errors in the next stage anyway, if the user supplies some nonsensical index vector.