add header and footer arguments to navbarPage
remove condition attribute from tabPanel

the tab content visibility needs to be the union of data-display-if and whether the tab is active (the currenty implementation makes it always visible when data-display-if evaluates to true)
Merge branch 'feature/navbar-header-footer' into feature/layout-cleanup
Merge branch 'feature/remove-tab-condition' into feature/layout-cleanup
remove panelWithSidebar (this scenario will be handled by more composable top-level pages)
rename tabsPosition to position
update NAMESPACE For removed panelWithSidebar
add fluid parameter to navbarPage to specify fluid vs. fixed layout
Merge remote-tracking branch 'origin/master' into feature/layout-cleanup
Merge remote-tracking branch 'origin/master' into feature/layout-cleanup
Merge pull request #320 from rstudio/feature/layout-cleanup

misc cleanup of new layout functions