Improve api-test widget
Setup a fake downloader for the proxy when running WirecloudSeleniumTestCases
Fix chromedriver bug
Import deleteWidget only if wirecloud.platform is in INSTALLED_APPS
New methods install_resource_to_user, install_resource_to_group and install_resource_to_all_users
New decorator: uses_extra_resources.

This new decorator allows to add extra resources to a given test method of a
TestCase.
Use the api-test widget for testing the MashupPlatform.http.makeRequest method
Some clean ups