Fixed class autoloading, using PSR-4

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Related tickets | 
| License       | MIT

We should use PSR-4 because `target-dir` field is now deprecated on Composer.

From the composer documentation v1.0:

target-dir

> DEPRECATED: This is only present to support legacy PSR-0 style autoloading,
> and all new code should preferably use PSR-4 without target-dir and projects
> using PSR-0 with PHP namespaces are encouraged to migrate to PSR-4 instead.