Following a suggestion by Austin in id:20130915153642.GY1426@mit.edu
we use remap for the over-riding bindings in pick. This means that if
the user modifies the global keymap these modifications will happen in
the tree-view versions of them too.
[tree-view overrides these to do things like close the message pane
before doing the action, so the functionality is very close to the
original common keymap function.]
We want to load notmuch-tree when notmuch is loaded, so include it as
a require in notmuch.el. To avoid circular dependency we need to move
one keybinding from notmuch-tree.el to notmuch.el: it makes sense for
it to be defined there anyway.
Since tree is now loaded by default there is no need to print a
message when it is loaded.
