Parametrised intervals

Interval arithmetic rewritten to use parametrised intervals.
Tests pass.
I prefer `floatinterval`; `float64interval` just looks awkard to me.

In Julia 0.3, there is the `float` function for conversion:
```
julia> float(1)
1.0
```
so we could just use this instead of `floatinterval(@interval(1,2))`.

Note that you can also do
`@interval(1, 2, Float64)` instead.