Invoicing updates

Added multiple new features and fixes to the invoicing gateway:

• Manual to avoid using direct debit directly
• CustomerByDocument to avoid passing the customer but use the mandate reference instead

Other changes include making the http layer an interface to improve testability

Bugfixes

Various fixes and improvements:

• Invoice can now be added directly (without ubl)
• Customer was missing a customerNumber
• Added rate-limited detail lookup for a document

Added extra documentation

Improved documentation + included license and fixes some linting issues

First go release

v0.1.0

Added missing webhook + README.md