2.7.0: S3 path creation

• Merged PR #21: Add com.amazon.aws.S3Key implementation to construct
  S3 paths, supporting arbitrary characters in file names.
  (@thekid)
• Merged PR #20: Add Resource::open() method to make URL creation simpler
  (@thekid)
• Fixed signing when URL contains encoded fragments; e.g. when uploading
  files with spaces in their names to S3.
  (@thekid)
• Changed Credentials string representation to use expiration in UTC.
  (@thekid)