Add `position` parameter to `navbarPage`

In response to a query by a customer via @garrettgman.

The only problem is that in the RStudio captive Shiny browser, the `fixed-top` and `fixed-bottom` options don't actually seem to work--not sure what's up with that. (Tested on Mac using v0.98.1049, the installed Safari is Version 7.0.6 (9537.78.2).)