Bring chapters 1-4 up to date

The security guide in places is pretty out of date. It even mentions Mongrels! I am sweeping through most of the words and dropping wordy stuff, updating code examples, clarifying things here and there. I have touched chapters 1-4 so far and intend to do the rest too.

I've replaced the diagrams featuring iMacs and PowerMac G5s with a simple sequence diagram. (I can easily change the output style, this is just default from websequencediagrams.com). I am open to suggestions as to the changes I made.

* Drop obsolete terminology
* Change the images to a flow chart style
* Improve sentence structure, style
* Mention Rails as secure by default
* Mention Carrierwave and Paperclip as secure by default

[ci skip]
Good point. I got carried away when I saw `attachment_fu`. Will fix in a new commit.