jetcd leaves behind non-daemon threads belonging to http client

Great project, but it's a little annoying that it leaves behind non-daemon threads that can keep a program from exiting or keep resources tied up.  In particular, if you use a framework which allows for cleanly shutting down *everything* without killing the VM, anything that leaves behind stuff like this is problematic.

I've patched jetcd to allow the HTTP client to be passed in, to avoid that situation.

(sorry about also including the ``pom.xml`` - I can't find a way to exclude that commit from my pull request)