Customize the generated Android project

Annotations:
* `app_name("My Name")` will be used to set the project full name and the shortcut name on Android.
* `app_version(1, 5)` or `app_version(1, 5, git_revision)` sets the project `versionName` in the manifest. It does not set the `versionCode`, this will require a way to track the project.
* `java_package` specifies the package of the generated APK (the package should be unique to each applications). The plan is to also use it to specify the Java namespace of the generated code.
* `android_manifest("some lines")` adds custom lines to the manifest. And  `android_manifest_application("some lines")` adds lines to the application sub section of the manifest.

I'll have to add this information as a manual somewhere. Should it be in the documentation of the `android` module or on the website?