Add non-parenthetical BSpline constructor?

This is mostly a matter of taste, but the current `BSpline` constructor has too damn many parentheses.

`interpolate(A, BSpline(Cubic(Line(OnGrid()))))` - heaven help you if you don't have bracket-matching turned on!

`∘(args...)` allows a more-legible syntax:

```
julia> Interpolations.BSpline(args...) = BSpline(∘(args...)())

julia> BSpline(Cubic, Line, OnGrid)
BSpline(Cubic(Line(OnGrid())))
```

If it's a good idea, I can put together a PR.