Add Configurable LogLevel specific to Scout Agent

[cschneid]

The Problem

Currently, we log all scout specific messages via the $logger parameter into the agent. But that can be awkward since you can't easily set the log level distinctly between Scout and the rest of your Application.

Proposed solution

Continue to take a $logger when constructing the agent. Output will be output there in the end.

Add a log_level config that takes the classic "debug", "info", "warn", "error" levels.

Add a ScoutLog class that all internal logging is routed through. It filters any messages noisier than the config that is set, and hands any left over to the provided $logger. If the config isn't set, pass through all messages.

Change all internal logging (in this repo, and also the -laravel repo) to use the new logger.

Document the new logging configuration & how it works in https://github.com/scoutapp/slate_apm_help/blob/master/source/includes/_php.md