Restructure folder structure to make the repository in-line with modern open-source repositories

[Viincenttt]

Right now the code is in the root of the repository. It makes sense to use a more modern folder structure where the code lives in the /src directory and the example project lives a /example directory. This makes it easier to differentiate between code of the Mollie API client and other types of files that live in the repository.

Suggested directories:

• .github
• src
• examples

Example: https://gist.github.com/davidfowl/ed7564297c61fe9ab814

[Viincenttt]

This is now merged in the development branch