Pass version_id in copy if key is versioned

The current key.copy function grabs the current version, ignoring the key's version_id if one has been specified.
Add support for SQS PurgeQueue operation.
Added suppport for additional fields in ClusterSummary and Cluster objects
Due to batching an empty result does not imply there are no results left. Consider a table containing 100k items, a batch get is executed for 10k items. Of those 10k items about 50% do not exist. If a single batch request of 100 items (the DynamoDB limit) does not contain a result then no further results will be processed.
Added test to cover empty batch result in a batch_get operation. All keys should be processed rather than returning on first empty batch result.
FIXING SES get_identity_dkim_attributes fails when input length > 1

boto.ses.connection::get_identity_dkim_attributes would call
_make_request() but would incorrectly parse the XML response from AWS.
If an input list with length greater than 1 was provided, this method
would only return a response for a single list element.

Adding DkimAttributes to list_markers and updating unit tests.
Correct join syntax

In Python, join is a method on string, not on list.
Added ELB Attributes to docs
Merge pull request #2806 from zvickery/develop

Add support for SQS PurgeQueue operation. Fixes #2806.
Adding unit test for boto/boto#2803
Merge pull request #2803 from sblosser/patch-1

Pass version_id in copy if key is versioned. Fixes #2803.
Merge pull request #2807 from dennisatplumbee/develop

Added suppport for additional fields in EMR objects. Fixes #2807.
Merge pull request #2809 from serviceagility/develop

DynamoDB table batch_get fails to process all remaining results if single batch result is empty. Fixes #2809.
Merge pull request #2810 from monkeysecurity/develop

Fix SES get_identity_dkim_attributes when input length > 1. Fixes #2810.
Add test for _sandboxify
Add missing comma
Merge pull request #2817 from bhagany/patch-1

Fix bug by using correct string joining syntax. Fixes #2817.
Merge pull request #2821 from micahhausler/documentation/elb

Added ELB Attributes to docs. Fixes #2821.