pip install - Throw errors if dependencies not found

[AbdealiLoKo]

pip install works even if some of the main dependencies like java are not found.

Currently the package asks for the zxing java library's path. Which isn't very helpful for people who wish to make packages out of this. (We can't as the user to tell us the xzing path, because they won't know !)

It would be better is the setup.py downloads the appropriate files itself and uses these downloaded files when it needs them. Otherwise, the installation of this package and it's dependencies is too be complicated to be used in python projects.