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

Add extension_manager.py to download online extension repo.
Usage:
extension_manager.py
--add=https://github.com/shawngao5/crosswalk-android-extensions.git
--name=iap --project=/home/xgao/tmp/iap/
Downloaded extensions are saved under <project>/extensions/

This tool also can remove, list and search local extensions.

To package these extensions into app, full path is not neened. Developer
just has to provides the name of extension, such as "iap", to the --extension option
of make_apk.py.

BUG=XWALK-1947
A link break is added. But github doesn't fold this block.