New Ipo and IpoCurve API. Ipo and IpoCurve objects support the [] operator,
for accessing the curves of a particular Ipo and for accessing the value
of an IpoCurve at a specific time. Both modules were also "tp_getset"-ified.
Also, code for an alternative method (Antont wanted this) of accessing curves
via Ipo attributes is included for now, for people to try and see if it's
actually preferable to the Ipo [] operator.
These are all new additions; nothing was intentionally removed from the API.
If you find the something in the existing API has changed, let me know.
