libevent integration

[tohava]

I've implemented a small piece of code that allows integrating libstrophe with libevent. It adds a new function
xmpp_conn_t *xmpp_conn_ev_new(xmpp_ctx_t * const ctx, struct event_base *base) that creates a connection that uses libevent loop instead off libstrophe loop.

The code is still missing at least two crucial parts:

• Error handling
• Allow compilation switch to disable/enable libevent support.

It might also be wise to somehow generalize it to allow other adapters except for a libevent adapter but i'm not 100% sure about how to do it yet.

Here is a commit with my changes, it is not complete yet so I'm not doing a pull request.

tohava@6e62612

Is what I have so far ok?
Also, is this fork of interest for you, will you consider merging it?