API/ENH: create Interval class
xref #8595

something like: `Interval(left=(-1.5,'open),right=(0,'closed'))`

reprs to `(-1.5,0]`

can be used immediately in the index values for `pd.cut` and such (as an object index)
evenutally can form the basis for `IntervalIndex`

cc @shoyer
cc @JanSchulz
cc @rosnfeld
