[Android] Add new command for make_apk.py to package external extensions 

Add extension_manager.py to download online extension repo.
Usage of extension_manager.py:

1. Add extension
$ ./extension_manager.py \
--add=https://github.com/crosswalk-project/crosswalk-android-extensions.git

2. List all extensions
$ ./extension_manager.py -l

3. Search extension
$ ./extension_manager.py --search=crosswalk*

4. Remove extension repo
$ ./extension_manager.py --remove=crosswalk-android-extensions

5. Enable extension
$ ./extension_manager.py --enable=crosswalk*

6. Disable extension
$ ./extension_manager.py --disable=crosswalk*

Usage of make_apk.py
$ python make_apk.py --package=your.app.name --manifest=\<your_manifest_path\>

BUG=XWALK-1947
What's `item.json` here? is it `build.json` in you comments above?