Make create_pile_with_data accept POST form body

[MusikAnimal]

It appears the API only accepts pages via GET, meaning you can easily get a 414 URI Too Long error, even with just a hundred pages, let's say, if the titles are really long. If you accept a POST request we can transmit more data. Since you're creating a resource with this endpoint, I think it makes more sense to only respond to a POST request anyway, per https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5 GET requests should be "safe" and idempotent.

Thanks for the otherwise awesome PagePile service :)