README revamp
README revamp

* Make it clear what this plugin actually does up-front; it has become much more than just j2objc translation.
  For users of the plugin, the execution of J2ObjC is just one implementation detail really.
* Installation instructions are in the plugin already.
* Why Gradle 2.4 (vs others) is needed is also an implementation detail.
* Mentioning GWT is irrelevant to most people.  We could just as well mention web service implementation in Java, etc.
* Show a more normal shared/build.gradle - nearly everyone depends on *some* library.
* Use bullets to break up long text
maxbritto/j2objc-gradle,madvay/j2objc-gradle,madvay/j2objc-gradle,maxbritto/j2objc-gradle,j2objc-contrib/j2objc-gradle,maxbritto/j2objc-gradle,j2objc-contrib/j2objc-gradle,madvay/j2objc-gradle,j2objc-contrib/j2objc-gradle,madvay/j2objc-gradle,maxbritto/j2objc-gradle,j2objc-contrib/j2objc-gradle