Groovydoc + Non-android projects + SdkResolverTest fix + com.jakewharton.sdkmanager plugin alias

This pull-request accomplishes a few things:

+ It adds groovydocs to all classes in `src/main`, they can be improved, but I think it's a good balance.
+ It fixes a bug in `SdkResolverTest` that caused a few tests to fail if on a Windows OS due to `\\` vs `\\\\`.
+ It exposes `SdkManagerAction` as a standalone component that could potentially be used in a task, or somewhere else... `SdkManagerPlugin` simply constructs a `SdkManagerAction` and executes it.
+ Allows non-android projects to use all parts which don't use the `android` closure.
+ Adds `com.jakewharton.sdkmanager` as a plugin alias as should be done for newer gradle projects.