Get applicationId directly from the variant rather than parsing from the manifest

Here's the pull request implementing what I talked about in #19 

Tests appeared to run fine (assuming the command was `./gradlew test`). Here's the output:

```
[hsweers@Henris-MacBook-Pro gradle-play-publisher (master ✗)]$ gw test
:compileJava UP-TO-DATE
:compileGroovy
:processResources
:classes
:compileTestJava UP-TO-DATE
:compileTestGroovy
:processTestResources UP-TO-DATE
:testClasses
:test

BUILD SUCCESSFUL

Total time: 14.514 secs
```