Implement Behaviour for Smartsheet Client

This release removes the MockClient from the library, and instead implements a Behaviour on top of the client, which will allow library users to implement their own mock clients for dev/testing.

Release: Update Rows

Adds functionality to update rows

First release

Okay, there was technically a 0.1.0 before this one, but I forgot to tag that release. Then I made some tiny updates/improvements, mostly to the test suite, and decided this version was 1.0.0 🎉