updated for 1.3.0
Update CHANGELOG.md
Consistent usage of tabs for indentation
Clean up documentation, add DocBlocks for every method of `ApiClient`

Still needs more detail, but it will need to be provided by HelpScout
Clarify `model\Mailbox::toRef()` DocBlock
Merge pull request #6 from jeffbyrnes/clean-up-code

Clean up whitespace (consistent use of tabs), add more docs/comments
Merge branch 'master' into develop