The great dimension migration to Dim<[Ix; N]>

[bluss]

If you thought I was slacking, finally the rework of ndarray's dimension handling is ready (almost).

To have your code be compatible with ndarray 0.6.8 and with this and the future: Use the dimension type aliases. Use Array<X, Ix1> or Array1 for 1d arrays, Array<X, Ix2> or Array2 for 2d arrays and so on. Then transition will be very smooth.

• Tuples are still used to describe shapes when creating arrays (Array::zeros((3, 4))).
• The dimension values are represented using the new Dim struct. Dim<[usize; 2]> is a 2D dimension.
• Dimensions/Indices now implement indexing and arithmetic

Requires Rust 1.12 (macro expansion of tt).

Fixes#6

[bluss]

@SuperFluffy Serialization changes, now serializes dimensions as arrays instead of tuples. I think the defacto format for json remains the same in both serialization libs, but it needs checking (test are still passing). Handling that properly feels like a project of its own.