Fixes to support authentication using IAM roles + S3 uploads.

The `boto.utils.get_instance_metadata()` function was broken in Py3k, causing IAM Role authentication to break. Also, uploading with `set_contents_from_filename()` was broken due to binascii expecting binary instead of unicode.

I tested this patch under both Python 2.7 and Python 3.4, and it's compatible with both versions.