README update & Android-like adapter

Hello,

Recently I found your logback-android library and I was really inspired by how powerful it is !

I used it in several projects and now I have useful changes for logback-android:

1. **README update.**
There was an issue related to tag encoder of LogcatAppender (https://github.com/tony19/logback-android/issues/33). The solution was to use %nopex with the pattern of tag encoder. I've updated README and add some more examples of tag encoders.

2. **Android-like adapter.** 
I implemented simple Android-like adapter of slf4j's Logger. It seems to be very useful in small projects. Please take a look on it: [ch.qos.logback.classic.android.Log](https://github.com/noveogroup/logback-android/blob/master/logback-classic/src/main/java/ch/qos/logback/classic/android/Log.java).

It is possible I made a mistake in copyright notice of [ch.qos.logback.classic.android.Log](https://github.com/noveogroup/logback-android/blob/master/logback-classic/src/main/java/ch/qos/logback/classic/android/Log.java). I'm not so familiar with licenses - so please fix me if it is needed.

Regards, Pavel.