Speed up selectInput when there are many choices
Speed up radioButtons when there are many choices
Add tagSetChildren() and tagAppendChildren()
Add tests for adding children
Add tests for creating nested tags
Refactor tag()

This is much faster when there are large lists of children (and the code is
much simpler!)
Fix handling of empty tags
Refactor checkboxGroupInput
Don't use named list items for selectInput and radioButtons

The names aren't used anyway, and this matches previous behavior (Shiny 0.6.0)
In tag functions, drop NULL attributes
Export tagSetChildren and tagAppendChildren
Preserve attributes in child tags
Add tests for escaping in tags
Merge pull request #179 from wch/faster-tags

Faster tags