Always return the ID as an integer.
Always return an array.
Reset the object and return itself on save.
Use VCR for all tests against the API.

This way we have REAL data to test against.
The downside is, that my API key is saved here. I think I will revoke it
and create a new one for new tests (everytime I add new tests of
course).
Only parse returned body if it is JSON.
Only fetch the elements once.
Better documentation and fallback to super method.

DRY!, super does everything we need.
New resource InvoiceComment implemented.
Additional parameter actionkey for all search in InvoiceComment resource.
Add API key #1 for (published) tests.
Add contrib/.

Includes:

* badbill-repl.rb: pry shell with pre-loaded badbill client
* xml2json.rb: Convert XML into JSON as used with Billomat
Generated gemspec & added rake (needed for travis)
CI powered by travis
Specify versions in Gemfile
VCR macros are deprecated (and were not in use here anyway)
Decode gzip content.
Create invoice payments (to set an invoice's status to paid)
Raise error when API key or Billomat ID is not set.
List InvoiceComment in README.
Only send what's needed on #save