Add administration tutorial
Add system customization tutorial
Fix linkification of diffs "A..B" where A/B are SHA-1s

Linkifying A as a plain SHA-1 took precedence, so the diff reference
became two commit links to A and B with ".." between them.
Add basic linkification test
Only call LinkType.linkify() for words matching LinkType.fragment

This essentially moves this check out of each link type's linkify()
implementation, thus making the code simpler (and implementation of new
link types less error-prone.)  It also matches the documentation of
linkification customization in tutorials/customization.txt.
Support link types with leading non-word characters better

Requiring a word-boundary before a link "word" means that link types with
leading non-word characters must be immediately preceded by a word
character, which is unlikely to be the case.

Relax the requirement by requiring either a word boundary before the link
"word" or that the link "word" begins with a non-word character.  (Such
a relaxation was already in effect, but only for a select few non-word
characters.)
Add test for linkification customization
Fix use of super() in base.ImplementationError sub-classes
Add documentation/tutorials.txt advertising the tutorials