Timeouts & TTLs

Very much a work in progress, but attempts to put a deadline on all requests to Zookeeper. This will hopefully fix a number of suspected problems where calls to the library might never return.

 - [x] Switch logging to `seelog`, rather than `log` so we have more control over log levels
 - [x] Allow a custom request timeout to be set on a connection, defaults to `3s` which is probably far too high.
 - [x] Abandon requests which exceed the request timeout - applies to all requests
 - [x] Split apart timeouts and TTLs on Locks, allowing these to be set independently

Still to do
 - [ ] Per request timeouts