Add Modal closeButton prop

This prop when set to false will not render the closeButton. The default
is set to true.
Nav onSelect bugfix

The Nav and SubNav components were incorrectly accessing the child's
onSelect prop resulting in NavItem components inside a Nav not firing
their local onSelect function.
Check for the existence of document before accessing it.

Since document is referenced within getDefaultProps which unlike
componentDidMount is run in both browser and node environments we need
to make sure we check it exists before accessing it.
Stop title being transferred to the Modal div via transferPropsTo.
Merge pull request #69 from pieterv/modal-allow-no-close

Add Modal closeButton prop
Merge pull request #70 from pieterv/nav-onselect-fix

Nav onSelect bugfix
Merge pull request #71 from pieterv/node-render-fix

Check for the existence of document before accessing it.
Merge pull request #73 from pieterv/modal-title-fix

Stop title being transferred to the Modal div via transferPropsTo
Add missing Modal PropType of animation