POM classifier support

The Android resolver currently supports the `groupId:artifactId:version@packaging` format for defining Android dependencies, however as per [Maven's specification](http://maven.apache.org/pom.html#Maven_Coordinates) the POM can also have a `classifier` property. For example:
```xml

com.pollfish
pollfish
4.4.0
aar
googleplayRelease
compile

```
The above example is requested in Gradle by using `com.pollfish:pollfish:4.4.0:googleplayRelease@aar` (note the colon-delimited part before the type). It seems however that the resolver ignores the classifier when the dependency is being declared directly in an XML or when it is a transitive dependency.

Would it be possible to add support for it?

Thank you.