Added basic implementation of key/bucket versioning
Keep track of previous versions of keys
Implementation of bucket.list_versions
Merge pull request #143 from AsymmetricVentures/bucket_versioning

Added basic implementation of key/bucket versioning
Add NextToken element to appropriate SNS responses. cc #144
Merge remote-tracking branch 'upstream/master'

Conflicts:
	moto/sns/responses.py

Local responses.py is updated from remote