Added web.py proxy and updated interposition.json

Added a web.py proxy to test the WEBrick repository with TUF.
The web.py proxy listen to localhost:8080, while the WEBrick
listens to localhost:9294. Any 'gem' request to localhost:8080
is filtered using TUF and on success the file is returned to 'gem'.
Otherwise, the web.py proxy returns a HTTP 404 Not Found error.

Make sure you have virtualenv; activate virtualenv and then
install web.py and TUF using:
    source virtualpython/bin/activate
    pip install web.py tuf

Also, make sure that you have rvm with Ruby 2.0.0 installed.
Activate rvm and use Ruby is 2.0.0-p247:
    source ~/.rvm/scripts/rvm
    rvm use 2.0.0

Then start both servers (preferably in different terminals) using:
    ./tuf-proxy.py
    ./myserver.rb

Then on a third terminal remove all 'gem' sources:
    gem sources --remove http://rubygems.org
    gem sources --remove https://rubygems.org

When done, this should yield an empty list
(otherwise continue removing sources):
    gem sources --list

Finally, test the 'gem' repository with TUF:
    gem install [gemfile] --source http://localhost:8080
    gem update [gemfile] --source http://localhost:8080