Add recursive dependency fetching to the Makefile

Also small other changes like how we build the PLT.
Add cowboy_websocket:close_code/0

Also fixes a warning.
Add cowboy_bstr:capitalize_token/1

For optional header name capitalization. See the guide section about it.
Get the basics of Websocket covered in the guide
Explain how to write custom protocol upgrades in the guide
Do not use tabulations in the guide
Add hooks documentation to the guide
Fix capitalize_hook example
Finish the routing chapter of the guide

Note that this is the new routing not yet available in master.
The code should follow in a few days.
Add plain HTTP handlers documentation to the guide
Beginning of request object chapter in the guide
Set max_keepalive default to 100 instead of infinity
Remove a few old @todo comments
Document loop handlers in the guide
Finish up the Internals chapter
Document the static handler in the guide