Fixed issue with pagination in S3.

The next_markers method in object_collection.rb for S3 did not reference the
"next_marker" property, causing issues when the contents.last value was
missing.

Added Cucumber/RSpec tests to cover this case, added a check for next_marker
to object_collection.rb, and refactored the marker selection logic to be more
readable.

Fixes #247
Merge pull request #274 from aws/issue247

Fixed issue with pagination in S3.