Finish S3 Blobstore implementation

[iorubs]

Description
The code is almost identical to the existing File BlobStore implementation (in fact it has already been added to nexus/blobstore_s3.go)

The problem is that testing that code is a bit awkward, because it requires valid creds connecting connecting to S3.

We already have a mocking test suite in place and a bunch of api mock examples (look up test with MockedClientSuite). So we just need to come up with the mocking data and test code.

Acceptance:

• Enable s3 api methods
• Using the nexus api (http://localhost:8081/#admin/system/api) manually perform all the different tasks and store the results. (note this requires valid AWS creds).
• Create tests using mock data based on the sults from the previous step.