Check for local credentials when constructing builder

[kylebarron]

I first tested constructing an ObjectStore Python object to an s3 bucket on my local dev machine, but it didn't work out of the box because it wasn't able to automatically infer the credentials from ~/.aws/credentials.

Boto3 checks the local credentials file automatically and it would be nice for object-store/object-store-python to do it automatically as well.

Would you accept a PR for this?