Handle Firefox Account creation. (#30)

This pr is for issue #30, please keep in mind that is WIP pr and I have to rebase it after pr #32 get's landed.

Notes:
I packaged the fxa-python-client and I install it with `pip install fxa-python-client-1.0.0.tar.gz`, so I don't have to do other hacks (to move over the setup.py location and to return, to install git on slaves or to clone the workspace from another job and to exlude the trigger.py when removing the repository)
The tradeoff of this was that I had to make a requirenmens text file and install them with pip install -r

Because we have to add the credentials in config file before ve have an environment, I declare the at the top of the script and create the account just before I ran the testrun. If an exception is thrown and we fail to create the account the build will be aborted, so this is safe. 

For a uid at emai creation I made use of BUILD_TAG variable because is the only one is unique, the BUILD_ID is the timestamp and when running concurrent builds it might duplicate.
https://wiki.jenkins-ci.org/display/JENKINS/Building+a+software+project#Buildingasoftwareproject-below

Given that we download the tests into the workspace directly, we should also do this for the build. Not sure why we use `builds` here.