Support classifier

[usr42]

Support classifier and extension in dependencies, both in string and map notation.

Details about classifier: https://docs.gradle.org/current/dsl/org.gradle.api.artifacts.dsl.DependencyHandler.html#N16F49

[patrikerdes]

I'm not sure if the build.gradle changes were intentional or a mistake, but I did not want them so I did a commit to revert the changes to that file.

[patrikerdes]

There were codenarc issues, I fixed them in your branch.

Please always run ./gradlew build and make sure it is successful before creating a PR.

[usr42]

You are right, the build.gradle change was not intended. Sorry for that and the codenarc issue. I'll run the checks the next time.

[patrikerdes]

No worries.

I'm seeing a lot of failing tests both on Travis CI and when running locally, would you mind having a look at that?

[usr42]

I've fixed the failing tests. ./gradlew clean check with SKIP_SLOW_INTEGRATION_TESTS ran successfully. I'm currently running without SKIP_SLOW_INTEGRATION_TESTS.

[patrikerdes]

LGTM