Usability improvements

1. You no longer have to subclass ApplicationContext in order to use fbs. Instantiating it is enough. See the new default main.py generated by startproject.
2. Previously, fbs always included boto3 as a dependency. That is, when you did pip install fbs, then boto3 was installed as well. This however is only required when you use fbs's release or upload commands. This release changes it so boto3 isn't installed by default. If you do want to use release, you need to pip install fbs[upload].
3. fbs now declares PyInstaller as a dependency. So you no longer have to do pip install fbs PyInstaller==3.4. It suffices to pip install fbs.