Update naming and add namespaces

[TerenceJefferies]

🆕 Update the entire package to support namespacing.

🔧 Remove support for anything under PHP 7.1

📁 Separate code into class files.

[tomhazledine]

Hi @TerenceJefferies, thanks for this PR. Namespacing has been on the to-do list for a while, so really great to see this added. And we're all glad to see the back of 5.6 😄Your refactoring of the codebase is a big step forward.

Couple of requests before merging, though:

1. Your changes are breaking the CI. Would you be able to take a look? Everything works okay locally (tests pass, code runs, etc.) but it would be great to fix whatever the issue is here
2. There has been a new release (v0.29.0) and it would be great to have those additions merged here.

[TerenceJefferies]

Hey @tomhazledine no problem at all, I've sorted all of that out now, merged the latest changes and sorted out the build 👍

[tomhazledine]

Thanks for the great work @TerenceJefferies - feels good to push this SDK over the line into v1.