Fixes for issue #7, #8, #10 and #11

[unicod3]

Hi,
First of all thanks for this great repo.
I tried to initialize the formset in a modal but I had this problem that when I first open model and click the add button everything works fine but then if I don't refresh the page and open another modal with formset and click the add button it added more than one form(in the numbers of models that i open and close).
That was a strange issue and I found that if I add off method before the on method issue fixed. #11

[unicod3]

Later on, I also had to add the fix @truhlik suggested on #7 for my project

[unicod3]

Well, this will also fix #10 with the commit 3 but I think #10 can be solved with a better approach.

[unicod3]

Well, I also write a method for issue #8
Now it hides the last item's delete button, and if you click the add button it shows the delete button again.

[truhlik]

Hi @unicod3 thank you for these commits.

I have tested and found one bug. I found that DELETE button is completely missing in all forms in formset. But after clicking on ADD button then DELETE button appears everywhere.
I check your code, but don't see any problem. Could you fix? Thank you.