Complete non-blocking (async) parser implementation

[cowtowncoder]

Now that API has been defined, and most of parsing functionality done (with character data, elements, PI, comments, xml declaration), need to complete rest of functionality. What remains is:

• DOCTYPE declaration (only internal subset, same as blocking impl)
• Coalescing mode (to degree feasible)
• Parsing of (but not expansion) general entities

[cowtowncoder]

DOCTYPE handling has been implemented by 0.9.7; entities as well. No coalescing mode; will create a new issue for it, separately.