Memory management help

[jessegrosjean]

I know this lib isn't advertised as ready for production, but I've been having fun using it in a side project.

I've also been making some fixes as I run into issues:

#14

In my current use it seems the parsed libxml2 structures are never freed. I see that it's not as simple as calling xmlFree upon deinit of swift object since the xmlPtr's are not tied to swift object lifetimes. I'm unsure of best way to solve.

Do you have a memory management design in mind so that libxml2 structures are freed?