Streaming XML parsing

I started work on implementing streaming XML parsing. There is still a lot of work to do but I really would appreciate early feedback so that I can develop in the right direction.

Very basic usage which highlights my current architecture choices:

```Julia
using LibExpat

cbs = XPCallbacks()
cbs.start_element = function (h, name, attrs)
    @printf("Found start tag: %s\n", name)
end

xp_streaming_parse("<t>t</t>", cbs)
```