Custom Logger

Hi Sam and other contributors,
Great job. Keep it up!

I have a feature suggestion. Would be nice to be able to provide a log listener and a log level. It's useful in cases when you want to collect and send logs to the server, and you use for this a custom logger. So it can be something like this:

``` java
Glide.get(this)
.setLogLevel(FULL)
.setLogListener(new LogListener() {
@Override
public void log(String message) {
}
}
```

Or maybe something more precise:

``` java
public void log(LogLevel level, String message)
```

Start by locating Glide.get and the existing logging implementation and configuration points. Review the issue's proposed LogListener and log-level APIs; done means callers can configure both and receive log messages with the selected level, with tests covering the new behavior.

Written by the indexing model from the issue text.