Add support for PSR-4

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -

PSR-4 has garnered enough votes to be supported so this is added for shorter module paths.

You will need to run `composer self-update` and `composer dump-autoload` for this to work.