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.
~~~
16:56:43 CROSSWEAVE INFO: Wiping data from server.
16:56:46 CROSSWEAVE ERROR: getSignedInUser() failed with: {}
16:56:46 CROSSWEAVE ERROR: [phase-1] Exception caught: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIObserverService.removeObserver]"  nsresult: "0x80004005 (NS_ERROR_FAILURE)"  location: "JS frame :: resource://tps/tps.jsm :: TPS__observe/< :: line 161"  data: no] Stack trace: TPS__observe/<()@resource://tps/tps.jsm:161 < TPS__observe()@resource://tps/tps.jsm:160 < canQuitApplication()@resource://tps/quit.js:18 < goQuitApplication()@resource://tps/quit.js:31 < TPS__quit()@resource://tps/tps.jsm:268 < TPS__DumpError()@resource://tps/tps.jsm:137 < _executeTestPhase()@resource://tps/tps.jsm:741 < <file:unknown>
16:56:48 Downloading: firefox-33.0a1.en-US.mac.tests.zip 
16:56:48 Downloading: firefox-33.0a1.en-US.mac.dmg 
16:56:48 Command '['runtps', '--binary', '/Users/danielapetrovici/workspace/mozilla-central_fx-account/build/FirefoxNightly.app/Contents/MacOS/firefox']' returned non-zero exit status -9
16:56:48 Build step 'Invoke XShell command' marked build as failure
16:56:48 Finished: FAILURE
~~~