Add support for path-oriented diagrams.

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=26. Original issue from fryguy...@gmail.com on April 30, 2011, 03:07:20 PM UTC)

I don't know if path-oriented is the best name or not but on the simple end of things this is arrows at the end of paths. On the more complex end of things is custom stroking. Common to both of these ends is orientation at any point of the path (first derivative with respect to the parameter in the case of a segment).

[fryguybob]

(Imported. Original comment by byor...@gmail.com on August 16, 2011, 07:22:07 PM UTC)

I'd like to at least get arrowheads into the 0.4 release. Custom stroking can probably wait a bit longer.

[byorgey]

@fryguybob, I must admit I am still not sure what exactly this ticket is about. Perhaps we can close it and convert it into several more specific tickets, now that we have some better ideas about where we want to go with paths?

[fryguybob]

Indeed. At some point I might have a clearer vision for what I would want and I'll open a new ticket.