Add docstrings to stanzas, docs for xep0030 plugin
Fix typo and resulting AttributeError in Client
Fire connection_made callback
Provide tools to register for IQ requests
Rewrite how handling IQ requests works

We are now using coroutines, one per IQ request. This allows for
parallel processing of IQs (which is generally desirable) and more
flexiblity.

Coroutines are the most abstract object we currently have: It can be a
normal function or be a real coroutine. Thus, any primitive can be used
with a coroutine, with the usual asyncio contract (don’t block for too
long).
Implement preliminary XEP-0030 support and add docs