Access messages through mail store interface

Access messages through mail store interface

This patch modifies notmuch binary to access the messages through mail
store interface, so that non-file based mail stores can also be
implemented.

The API of notmuch library was changed. Now,
notmuch_message_get_filename() returns relative file name with respect
to the database path. As a result, notmuch show also outputs relative
paths so that MUAs need to be changed.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>