EvilYoda
Lifer
You can't uninstall them without root access (AFAIK), but you can install any app you want and set it as default instead and you shouldn't run into the stock calendar or messenger unless you specifically open them.
I think I read something about Lollipop that all programs, including carrier-installed apps, have to go through the Play Store, meaning that the ones you don't want can be uninstalled. I don't remember the specifics, but that'd be awesome.