Handle concatenation of Note and Rest to NoteSeq

This was bothering me.

Now you can do:

    >>> s = NoteSeq('C B D')
    >>> s + Note('C')
    <Seq: [<C>, <B>, <D>, <C>]>

    >>> Rest(4) + s
    <Seq: [<R: 4>, <C>, <B>, <D>]>