Retry SimpleDB 503s, honor NextToken when present

This change incorporates several modifications to `erlcloud_sdb` to bring it closer to Amazon's documentation:

  * Retry SimpleDB requests [if a 503 occurs](http://docs.aws.amazon.com/AmazonSimpleDB/latest/DeveloperGuide/APIUsage.html#APIErrorRetries), repeating the request until it succeeds or times out, waiting longer between each request
  * Return the [`NextToken`](http://docs.aws.amazon.com/AmazonSimpleDB/latest/DeveloperGuide/SDB_API_Select.html) from `erlcloud_sdb:select` so that it can be used in subsequent requests
  * Add `erlcloud_sdb:select_all`, which automatically requests all result pages and returns the complete result set

The changes have unit tests.