Use VCR instead of mocking

[matthewhanson]

The vcrpy library is a great alternative to mocking, since it allows tests to be run against real endpoints and just records the result for later testing. These cassettes can then be used instead of making real calls.

Basically it allows for auto-creation of mock data. #43 shows an example test using VCR for downloading a metadata asset.

I think this can be used in place of moto for s3. If so then tests should be performed against public data without requiring authentication.

Mocking may still be required for some tests, such as testing uploading items.