WIP: Order (and series) at point != 0 and oo

n.b.: I'm not sure about the syntax for Order.  Perhaps, an iterable container of pairs ((var1, point1), (var2, point2), ...) could be better (like in subs method) for multivariate O symbol.  Yet, it's not a backward-compatible (since 0b13b0f we have Order at oo, not just in 0).

TODO:
- [ ] cleanup \__new__ and _eval_subs methods
- [ ] rebase