Python script for triggering tps testruns on jenkins jobs. (#17)

This is for issue #17.

What the script does:
* Checks and downloads the build and packaged tests.
* Unzip the packaged tests.
* Call create_venv.py to create an environment and install tps
* Activate the environment
* Import mozinstall, check if build is installer, if so, install it 
* Check mobile and debug options and create the command arguments for tps testrun
* Call the testrun

Notes:
I added the username and password as hard coded variables, they will be changed in issue#30
When calling create_venv.py we have to be in the same directory with the script becasue else it wont find the 'config/config.json.in', data_files element declared in setup.py.
I trigger the processes with check_call and catch the exceptions, then if an error occured I return the script with return code 1 so the build will be invalidated.
I made a download method in favor of urllib.urlretrieve because the later doesn't throw or notify if the file doesn't exist but it will download the 404 page for instance.
Have you checked how that works? Do we exit with the right code? `sys.exit()` expects a number here, which I don't think is getting passed in. Also maybe in such a case a little error message would be good, so we know what happened.