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
functions for creating lower-level bootstrap layouts
move page functions to the top of the bootstrap-layout file
refactor functions to use fixed and fluid explicitly and to introduce the notion of "layout" functions
introduce more layout functions
add validation of layout function inputs
more revisions to layout functions -- go back to page level constructs for fixed and fluid
add fluid parameter to navbarPage to specify fluid vs. fixed layout
add hr tag to exported html builder tags
update NEWS
update NEWS
Merge pull request #324 from rstudio/feature/hr-tag

add hr tag to exported html builder tags
Merge remote-tracking branch 'origin/master' into feature/layout-cleanup
Merge remote-tracking branch 'origin/master' into feature/layout-cleanup