Fix PHP namespaces

[Beutlin]

Currently the namespace is inconsistent. The current directories are all lowercase, but the namespace itself starts with a capitalized letter.
In my opinion it should not be necessary to define each namespace in composer.json.

Possibility 1:
Rename directories and uppercase them application/Api (instead of application/api)

Possibility 2:
Lowercase first file in PHP namespace shaarli\api (instead of Shaarli\Api)

---

Which namespace will you prefer for tests?

• Shaarli\Tests\Api
• Shaarli\Api
• Tests\Shaarli\Api
• ...