Unmask websocket frames on the fly and optimize the code

No change in functionality, but this will allow us to validate
that text frames are utf8 without having to receive the frame
entirely.
Websocket text frames are now checked for UTF-8 correctness

The autobahntestsuite now passes 100% of the tests. We are
getting close to fully implementing the Websocket RFC.
Improve websocket close handling

We now always send a failure reason (bad protocol, bad encoding, etc.)
unless the closure was initiated by the client and it didn't send a
close code.

We now check that the close frames have a payload that is valid UTF-8,
unless they don't have a payload at all.

We now do not crash the process anymore when bad opcodes are sent, or
when the opcode 0 is sent before fragmentation was initiated.

Overall this makes us closer to full compliance with the RFC.
Add Sec-Websocket-Protocol header parsing
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