Add files from tripit/slate

at commit c665bb4e8ff10743bf4c5ef5cbfea40abbfba391
Make customisations
Add content
Fix error analytics code
Make ad responsive
Add documentation for dropzone options
config.rb: enable livereload
Update image resources
Use summary_large_image twitter card
Modify the Snap Function targets section
Add Snap Relative points section
Remove mention of 'gestureinertiastart' event
Add code sampeles for restrict
Improve snap docs
Close fenced code block for restrict elementRect
Use *.md.erb extensions; partials in index.md.erb
Fix a rendering issue between actions and snap
Rename default layout to 'slate.erb'
Delete _errors sample include
Add a warning about pointer/InteractEvent sequence