text_entry_minibuffer_state $match_required renamed to $require_match
refactor completers
read-url-kill-item: new command

C-k in a read_url interaction deletes the selected completion from
history and/or bookmarks.
index-webjump.js: refactor (for recent changes to webjump system)
make require_match a propery of webjumps instead of completions

Nested completers present a hard problem with respect to the require_match
property, and where exactly this flag belongs when a nested completer,
like webjump_completer, needs it (in some cases).  As a UI concern, it is
clear that it breaks abstraction to use completions objects to carry this
flag from webjump to reader, and it necessitates overmuch plumbing to do
so.

It makes more sense to put the property on webjumps, because it is in a
very literal sense a property of the webjumps that a match is required.
But the reader doesn't know anything about webjumps, so in order to
communicate with the reader, we need to carry the flag from webjump to
reader via the completer.  This still needs explicit support from
merged_completer, and it is still a break of abstraction, but a lesser
one, and we eliminate fragile checks for the 'get_require_match' property
to dispatch special behavior.
remove obsolete checks for completions methods

Completions objects all have all of these methods now.
merged_completions: remove 'for_each' method

Only used for 'destroy'; no reason to abstract it.
webjump_completer.complete: take care of a js warning

a js warning seen in xulrunner 13:

  Console error: [JavaScript Warning: "reference to undefined
      property 0" {file: "chrome://conkeror/content/webjump.js" line:
      171}] Category: chrome javascript
match_webjump: deal with a js warning

warning in xulrunner 13:

  Console error: [JavaScript Warning: "reference to undefined
     property 1" {file: "chrome://conkeror/content/webjump.js" line: 108}]
     Category: chrome javascript
match_webjump: always return four values
shell_command_completer: update for new completer system

get_shell_command_completer, replaced by shell_command_completer
minibuffer-read-file.js: further work
tests/simple/completers.js: tests for completers