dynamo v2

This is the first release of dynamo v2, which uses aws-sdk-go-v2 (PRs: #206, #232).

For tips on migration from v1, check out this new section of the README: Migration Tips.

The API is mostly the same as v1, but there are some changes:

• All request methods take context now (e.g. OneWithContext is now One).
• Retrying is configured through aws-sdk-go-v2, see Retrying docs.
  • This changes the default behavior for transactions, to get the v1 behavior back use dynamo.RetryTxConflicts (see above doc for usage example).
• Arguments that took int64 (such as in Query.Limit) now take int instead.
• PagingIter.LastEvaluatedKey can return an error now (in rare situations when IAM is misconfigured).
• KMSMasterKeyArn renamed to KMSMasterKeyARN.

v1 may continue to see new releases for bugfixes and such: v1 branch. Please feel free to report issues for both v1 and v2. Note that AWS is going to end support for the v1 SDK soon, although we can expect that it will continue to work.

Thank you to everyone who contributed (especially @niltonkummer and @irohiroki who contributed to the v2 port), and to everyone who had patience for this long-awaited release.