Add CardView support library module

Fixes issue #106 
When I tried to launch a test using this module with ``minSdkVersion rootProject.ext.minSdkVersion`` I got the exception during a build:

```
Error:Execution failed for task ':app:processDebugTestManifest'.
> java.lang.RuntimeException: Manifest merger failed : uses-sdk:minSdkVersion 7 cannot be 
different than version L declared in library com.android.support:cardview-v7:21.0.0-rc1
```

Gradle tries to merge the manifest of the support lib with assertj module and the support lib forces minSdkVersion to L. 
Then I found the [article]( http://www.reddit.com/r/androiddev/comments/297xli/howto_use_the_v21_support_libs_on_older_versions/). It says that the new support libs (CardView, RecyclerView, Palette, etc.) are locked into the L API level and offers some hack to get them work on older levels. Should we apply this hack and how if a test manifest is generated?