ruby: add bindings for `notmuch_database_get_all_tags`

The Ruby bindings were missing a way to get all the tags of the
database. Now you should be able to access this with the public
instance method `all_tags` of your database object.

Example of use:
    notmuchdb = Notmuch::Database.new path, { :create => false,
    	:mode => Notmuch::MODE_READ_ONLY }

    my_tags = notmuchdb.all_tags

    my_tags.each { |tag|
      print tag
    }

    my_tags.destroy!

Amended by db: improve error reporting, add test

1 files changed, 8 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index c945c245..90e233bf 100644
--- a/NEWS
+++ b/NEWS
@@ -1,3 +1,11 @@
+Notmuch 0.23 (UNRELEASED)
+=========================
+
+Ruby Bindings
+-------------
+
+Add support for `notmuch_database_get_all_tags`
+
 Notmuch 0.22 (2016-04-26)
 =========================