King Reza: "Have" can be contracted even when used as the main verb; I think this is more common in the UK than in the USA. Here on the Web, Google finds no small number of instances of "I've a", "I've the", and "I've two cats".

Now the propriety of such usage is a separate question. Arguments over grammar often boil down to a question of "Says who?", and contractions are one of the murkier areas if you're looking for absolutely binding rules. It's true that they're best avoided in formal speech and writing, but otherwise if you want to contract, contract. I don't know of anyone with the authority to tell you you mayn't.