Incremental refactor

Following on from !144 (closed) (from book https://leanpub.com/mlaphp) I figured we could just go through step by step and keep track here.

These are all the chapter headers, some of them won't make so much sense, feel free to remove things :)

If the task needs more detail, can also create an issue and link it back here.

• Implement An Autoloader !157 (merged) thanks @tiltec!
• Consolidate Classes and Functions !269 (merged) thanks @NerdyProjects
• Replace global With Dependency Injection
• Replace new With Dependency Injection
• Write Tests
• Extract SQL Statements To Gateways - #9
• Extract Domain Logic To Transactions
• Extract Presentation Logic To View Files
• Extract Action Logic To Controllers
• Replace Includes In Classes
• Separate Public And Non-Public Resources
• Decouple URL Paths From File Paths
• Remove Repeated Logic In Page Scripts
• Add A Dependency Injection Container (added in !264 (merged))