core.async upgrade, configurable async timeouts & logging

Hi guys,

Here is another change I suggest for inclusion within capacitor. With this change, it is now possible to override the default get and post options, specifically, by defining a ```:timeout``` property within the client, inside either ```async-get-opts``` or ```async-post-opts```. The property differ from the synchronous client as httpkit does not accept the same parameters than clj-http.

I've also added a bit of logging around post-points. It will now log at warning level when some points couldn't be added to InfluxDB. There is also a debug log that will log the number of points that will be posted. Might be useful to diagnose problems.

I've also upgraded the core.async version to the latest one available for clojure 1.5.1 (the more recent versions requires clojure 1.6.0).

Let me know what you think of all my changes.

Thanks,
Reynald