Fix multiple add entity during single execute

[AngelOnFira]

This fixes #71. It changes how some of the backend for orders works, specifically it treats orders as items that should only be done once and then removed from the orders queue. This might have adverse effects if someone runs:

seeder.execute()
seeder.execute()
seeder.execute()

However, I would argue that this is not a good way to make use of this library. I'd appreciate feedback if there are any issues with this 😃

The tests I wrote display behaviour that would have failed without the fixes.

[coveralls]

Coverage increased (+0.03%) to 98.656% when pulling 4dd4402 on AngelOnFira:fix-multiple-add-entity-during-single-execute into 27deaa2 on Brobin:master.

[coveralls]

Coverage increased (+0.03%) to 98.656% when pulling 4dd4402 on AngelOnFira:fix-multiple-add-entity-during-single-execute into 27deaa2 on Brobin:master.

[Brobin]

This is great! Never even thought about this case. Thanks for the fix, and the test perfectly describes it