Make repo pip-installable

[joehalliwell]

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This pull request makes it possible to do:

pip install git+https://github.com/CompVis/taming-transformers.git

I have done this by:

1. Adding __init__.py to the module directories
2. Moving a couple of functions inside the taming package

[joehalliwell]

I've had this forked for a while for my own experiments, but thought that it might be a useful alternative to @bfirsh's work over at https://github.com/CompVis/taming-transformers/pull/81