Changed Mac OS X section

MacOS is now Mac OS X. Added "Installation using the Apache CouchDB native application" section (inspired by MySQL documentation), simplified MacPorts section. Who chose MacPorts in favor of the native app is because already using MacPorts, so he knows how to upgrade packages, make the selfupdate, etc. I think we just need to provide minimal information to install and run it as a service.
I don't know about Homebrew, but I'm sure you don't need all those steps to install it. Someone should revise the section.
Moved also MacPorts as the second choice.