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
update NEWS
merge from master

Merge remote-tracking branch 'origin/master' into feature/bootstrap-layout

Conflicts:
	NAMESPACE
	NEWS
Merge remote-tracking branch 'origin/master' into feature/bootstrap-layout
update roxygen docs for navbarPage fluid argument
allow row functions to take a class
fix columnLayout function
Merge remote-tracking branch 'origin/master' into feature/bootstrap-layout
use paste0 where appropriate
allow pullLeft/Right functions to take tags or lists of tags; validate that all inputs are tags
prevent long line lengths for links to bootstrap docs
Rd changes for R CMD check
Automatically concatenate multiple class attributes in tag definitions
concatenate duplicate attributes in tag definitions
add support for menus on navbars (navbarMenu function)