Introduce the release.py script for MAS

[zugaldia]

The script is based on the SDK version, but adapted for MAS where a few steps don't apply.

Usage is however equivalent:

$ python release.py --help
Usage: release.py [OPTIONS]

Options:
  --stage [snapshot|beta|final]  The release stage.
  --version TEXT                 The version you want to publish. E.g:
                                 2.0.0-SNAPSHOT, 2.0.0-beta.1, or 2.0.0. If
                                 you set the version to "current", the script
                                 will default to the current SNAPSHOT version.
  --help                         Show this message and exit.

Examples:

• Publish a snapshot from master (release.py uses the current branch)

$ git branch
* master
  1234-fix-crash
$ python release.py --stage snapshot

• Publish a snapshot from a feature branch (same as before, just switch branchs with git):

$ git branch
  master
* 1234-fix-crash
$ python release.py --stage snapshot

• Publish a beta from a pre-release branch:

$ git branch
  master
* release-android-420-beta1
$ python release.py --stage beta --version 2.0.0-beta.1

• Publish a beta from a release branch:

$ git branch
  master
* release-android-420
$ python release.py --stage final --version 2.0.0

I tested with a SNAPSHOT and it was 👍 .

Fixes #218.