Only create asset folder when running as main script.

This commit makes it so that merely importing server.py doesn't cause the assets folder to be created, something which might be undesired in for example unit tests.
Unused imports.
Fix unit test for get_playlist.
Document how to run the unit tests.
More tests. Simplify code.
get_assets() is deprecated and no longer needed.

fetch_assets() performs the same function with more flexibility and should be used instead.
Merge branch 'master' into screenly-ui

Conflicts:
	requirements.txt
	server.py
Add "Add Asset" button.
Add Bootstrap libraries for animation and modal dialogs.
Added upgrade script.
Merge branch 'master' of github.com:wireload/screenly-ose
Starting work on Add Assets modal
Updates for code readability and PEP 8.
Merge branch 'master' into screenly-ui

Conflicts:
	server.py