Add support for `beforeNotify` callback

https://github.com/bugsnag/bugsnag-java/pull/15

Example usage:
```java
client.addBeforeNotify(new BeforeNotify () {
    @Override
    public boolean run(Error error) {
        // Sets the `groupingHash` option
        error.setGroupingHash("hello");

        // Overrides the severity
        error.setSeverity("warning");

        // Modifies the user information
        error.addToTab("user", "id", 1);

        // Prevents the error from being sent
        return false;
    }
});

client.addBeforeNotify(new BeforeNotify () {
    @Override
    public boolean run(Error error) {
        // This code will never run
        System.out.println("What is game night?");

        // The error will still not be sent ;(
        return true;
    }
});
```