Add integer field type

I did not like the fact that there was no integer field type. Also, I saw that you could add custom field types for forms, so I did.

In issue #14, this helps to fulfill the need for a house number field that checks whether a number is indeed entered.

I also added demo data to demonstrate the functionality. I have manually verified that it also sends out correct values (i.e. without decimal) in the e-mail confirmation.