nitg: Added 3 annotations related to API level target and support for Android projects

It enables to add the minimum, maximum and target API level annotations in android projects. The annotations are as follows : 
~~~
min_sdk_version(Int)
max_sdk_version(Int)
target_sdk_version(Int)
~~~

The target is the API used to build the project, the minimum sdk version specifies the minimum android API needed to run the app and the maximum is the maximum API level that can run the app. This PR allows to build for all android platforms instead of API 10 and below only. (API are platform versions)

The tests are structured to conform to #568. When #568 will me merged, the tests will have to be imported in `complete_mnit_android.nit`.
Won't compile before #580 is merged.