Releasing v0.16.1

Preparing 0.15 release.

Merge pull request SparkPost#29 from SparkPost/feature/issue_26_attac…

…hments

Feature/issue 26 attachments

Merge pull request SparkPost#25 from SparkPost/feature/betterSubstitu…

…tionSupport

Fixing issue SparkPost#24 `Substitution data as string`

Merge pull request SparkPost#23 from SvenWoltmann/master

When sending data to the API, data string should be converted to byte array only once

Updates README with simplified send email example

Updates readme with new version of library