loading_gif closes #67

[ConorSheehan1]

Closes https://github.com/IIIF/presentation-validator/issues/67 Would also recommend closing https://github.com/IIIF/presentation-validator/issues/69 first, otherwise the page looks like it's loading forever when it can't reach the manifest.

~~Also, is the gif licence ok? It's under cc-sa, found on wikimedia. If not I can try to find something fully cc-0?~~

Use cc-0 loading gif from https://loading.io/

[coveralls]

Coverage decreased (-0.4%) to 83.2% when pulling f673f9aac608d8f460a3bd61c773d19f9dc5fa5a on ConorSheehan1:loading_gif into e6207809ad17ed4b7a94b2035005c7bfc451e4b6 on IIIF:master.

[ConorSheehan1]

python 3.3 build failing to install lxml. This lxml version requires Python 2.7, 3.4 or later.

Also issue with pyld (dependency of iiif_prezi) which uses asyncio, since it wasn't included in stdlib until python 3.4.

async def async_loader(url):
            ^
SyntaxError: invalid syntax

python 3.3 is at EOL anyway, so might make sense to drop support? https://www.python.org/dev/peps/pep-0398/#x-end-of-life