Logging Hooks

[tgockel]

The C client's logging leaves a lot to be desired. Since log_callback_fn is simply void (*)(const char *message), we do not have a way to do basic things:

• Quickly post-filter by level
• Use a void* or other user data
• Transform the output in any way that requires context

We probably just want to work around this...or fix the C API's logging.

[tgockel]

Pushing this to v0.2 so we can think about configuration as a whole.

[tgockel]

I opened ZOOKEEPER-2969 in an attempt to get the C API changed to something a bit more usable, but no response. There's some argument to copying the C code and using my own internal copy. They're both Apache licensed, so that's A-OK.

[tgockel]

Pushing to 0.3 because ZK C API...