Add flag to make_webapp.py to specify build tool with the download url
User can specify build tool with the download url
Such as:
python make_webapp.py --version=2.31.27.0 --url=https://download.01.org/crosswalk/releases/android-x86/canary
Merge pull request #34 from kurli/download-url

Add flag to make_webapp.py to specify build tool with the download url