Move initialization of the blobstore provider from the provider's kotti_configure into Kotti itself.  This elimintaes the need to implement the same generic code in every provider over and over again.  It also allows to configure the blobstore with a single option only.  Update documentation accordingly.  Add YURL as a dependency / requirement.
Merge pull request #324 from disko/feature/blobstore_init

Move initialization of the blobstore provider from the provider's kotti_configure into Kotti itself