Add support for orders & invoice

[viezel]

• Add support for orders & invoice
• phpunit and unit tests

[viezel]

@sabas ready for review

[davidvandertuijn]

Quick suggestions:

• Reference to unece.org documentation instead of stylusstudio.com
• Use mixed in @param mixed $var when type is unknown ?

[viezel]

@davidvandertuijn sure ill change the reference.
Regarding the mixed - well if you ask me it would rather go full strict type, but maybe to big of a refactor. So where do you want the change?

[sabas]

Thank you @viezel, I'll try to merge this night..

Just to ask also to @davidvandertuijn I pulled the changes in my client code some days ago and I noticed it failed with a type error when providing arrays with ints as it was annotated with string in the parameters, so I changed the default values from null to the previous value as string (15th march commit)

Do you want access to the repository or to the org?

[viezel]

Sure @sabas That would be neat to gain access. Then I could setup travis to automatically run tests for this

[viezel]

@sabas thanks for merging it in.
can you grant me access to the repo, so I can tag the release so we follow semvar.

[sabas]

@viezel done please try