- added class `SIPMetadata` to be able to have multiple metadata
- added attribute `archive` (boolean) to say if the content should
  be archived or not

- updated the model views to integrate the new fields
- added the model view for SIPMetadata with links to SIP

- add a signal when a SIP is created from the API
- 2 API classes: SIP and RecordSIP to manage the models
  based on Zenodo:
  https://github.com/zenodo/zenodo/blob/master/zenodo/modules/sipstore/api.py
- add a function to automatically find the current storage
  location of a SIPFile
- add a config variable to generate agent of the SIPs
- updated tests