travis: Only notify IRC on success if the previous build failed

Without this, Travis is rather spammy.  Travis will continue to notify
the IRC channel on each build failure, which seems desirable.
emacs: Introduce notmuch-jump: shortcut keys to saved searches

This introduces notmuch-jump, which is like a user-friendly,
user-configurable global prefix map for saved searches.  This provides
a non-modal and much faster way to access saved searches than
notmuch-hello.

A user configures shortcut keys in notmuch-saved-searches, which are
immediately accessible from anywhere in Notmuch under the "j" key (for
"jump").  When the user hits "j", the minibuffer immediately shows a
helpful table of bindings reminiscent of a completions buffer.

This code is a combination of work from myself (originally,
"notmuch-go"), David Edmondson, and modifications from Mark Walters.
emacs: Expand default saved searches and add shortcut keys

This should help new users off to a better start with the addition of
more sensible saved searches and default shortcut keys.  Most existing
users have probably customized this variable and won't be affected.
lib: Improve documentation of _notmuch_message_create_for_message_id

Clarify the state of the returned message when
_notmuch_message_create_for_message_id returns
NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND.