Allow usage of a custom logger (Timber) instead of Logcat

[passsy]

#Log messages can now be forwarded to crash reporting tools (crashlytics)

Usage

RxBleLog.setLogger(new RxBleLog.Logger() {
    @Override
    public void log(final int level, final String tag, final String msg) {
        Timber.tag(tag).log(level, msg);
    }
});

[dariuszseweryn]

A great idea and addition. Just two thoughts:

• since it is enhancement of the API this should be merged to the development branch which will be merged to master on release of 1.4.0
• It would be good to an add appropriate note to README.md and CHANGELOG.md
  Otherwise it is a great feature and I am looking forward to merge it. 👍

[dariuszseweryn]

Start the line with a capital letter

[dariuszseweryn]

Maybe something like: Example of how to integrate with Timber:?

[passsy]

Rebased, added to feature to CHANGELOG and README.md

[uKL]

Dear Contributor,

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You can find a link here: https://cla-assistant.io/Polidea/RxAndroidBle