Updated Bucket.get_blob() and Bucket.delete_blob() to just take
a string instead of string or Blob. Logic being that if the user
already has a Blob, they can just call blob.delete() or
blob._reload_properties() (more convenient method needed).

Updated Bucket.delete_blobs() to check if each element in list of `blobs`
is a string or a Blob.

Updated Blob.delete() to call Bucket.delete_blob() with just a string and
updated Blob.rename() to call Blob.delete() instead of re-implementing the
mapping to Bucket.delete_blob().

Removed Test_Bucket.test_new_blob_existing, the last caller of
Bucket.new_blob that sends a Blob instead of a string.

Opting to just use Blob() constructor where relevant.