Update Maven version/instructions in README

[loisaidasam]

First off, thanks for the lib, it seems like exactly what I need. Onto my issue...

I'm a bit new to gradle, trying to figure out how to use this project.

I see the latest version in the [Maven Repo](http://search.maven.org/#search|gav|1|g:"com.wu-man" AND a:"android-oauth-client") is 0.4.5, and the samples require a version newer than 0.0.3.

When I tried to plug in compile 'com.wu-man:android-oauth-client:0.4.5' into my dependencies block I got an error saying that the lib depends on libraries but is a jar. Not sure exactly what this means, but after looking around on the internet I got the tip to append @aar to the end, so compile 'com.wu-man:android-oauth-client:0.4.5@aar' which seemed to work.

But then my further com.google.api.client imports no longer worked. I'm assuming that installing the aar version doesn't include dependencies or something? I'm working towards it.

[TimMackenzie]

Thanks loisaidasam for the tip to add @aar - that solved my issue.
With v0.4.5 as a jcenter dependency, I added the dependencies used in the library (with updated versions):

compile('com.google.api-client:google-api-client-android:1.20.0') {
    exclude group: 'com.google.android.google-play-services', module: 'google-play-services'
    exclude group: 'junit', module: 'junit'
    exclude group: 'com.google.android', module: 'android'
}
compile 'com.google.oauth-client:google-oauth-client-java6:1.20.0'
compile 'com.google.http-client:google-http-client-jackson:1.20.0'