Fix #2

When initializing the S3PO::File object, check to see if the bucket exists and -- if so -- assign it to a @bucket instance variable. If it does not exist, create it.
On gem installation, create the S3 bucket if it does not exist.

I have a concern about environment-specific creation, since it seems the bucket creation code is only run on gem install. Would we need to change this to iterate over the buckets and create them?
Merge pull request #7 from pubget/make_buckets_if_non_extant

Fix #2