Lots of small improvements

This release integrates lots of pull requests from the community. Thanks for the PRs, everyone.

• Iterators check if the context is canceled #166 (@jkawamoto)
• Allow endpoints to be specified by environment DYNAMO_TEST_ENDPOINT variables on testing #173 (@ken39arg)
• Housekeeping: #167 #168 #169 #170 (@mlafeldt)

Although this PR doesn't add any shiny new functionality, it introduces a new error called ErrNoInput that is returned if you use the batch/transaction APIs without providing it any input. See #174.