Move api requestor into a separate module
Remove keep, cropped, resized, __getattr__  from File

Unneeded tests were removed as well.
Remove is_on_s3, ensure_on_s3 from File #34
Rename File.file_id to File.uuid
Remove @property from methods that might do API requests
Remove File.serialize
Rename info method in ucare
Remove File.url
Rename storage_uri, api_uri, ucare, _info
Update is_stored(), is_removed(), filename()
Rename UploadCareException to UploadcareException
Add global conf
Make all api requestor's methods class method
Fix ucare_cli
Update api requestors (RESTClient, UploadingClient)
Add some docstrings
Move ``upload``, ``upload_from_url`` into ``File``
Rename file.py to api_resources.py
Update documentation

API reference updated. Deprecated things removed.
Make all imports relative