Allow use of S3 buckets in regions other than us-east-1.

Work around region issue in S3Backup.new_connection and
boto.storage_uri.  When attempting to connect to a bucket in a
non-standard (non us-east-1) region, boto complains

  S3ResponseError: 301 Moved Permanently
  The bucket you are attempting to access must be addressed using the
  specified endpoint. Please send all future requests to this endpoint.

References:

  https://github.com/boto/boto/issues/443
  https://github.com/heroku/WAL-E/issues/24
Address CR feedback.

In s3_endpoint_for_uri(), update the hard-coded region dictionary with
any entries in boto's S3 region map if available to maximise
compatibility.

Add a unit test and integration test.  This adds a testing dependency on
unittest2 which is a Python 2.6 backport of the unittest module
improvements in Python 2.7.

Having AWS credentials in your environment along with the
WALE_S3_INTEGRATION_TESTS (any arbitrary value) will trigger inclusion
of the integration tests.