Raise helpful error when using ExMachina.create functions

[paulcsmith]

I think it would be good to add helpful errors when calling create, create_pair and create_list on ExMachina so people know that things have changed when they upgrade to 1.0.0

[paulcsmith]

@davidkuhta is working on this one I believe :)

[davidkuhta]

@paulcsmith
Note: this is the error-helpers branch.

create functions with error-helpers

Tests split based on arity to match existing tests:
create/2, create_pair/2, create_list/3 tests
create/1 test

[paulcsmith]

@davidkuhta This is great. Thanks for working on them. Could you open a pull request for these two issues? That would make it easier to see what changes and give comments :D