Add insert and replace methods

Both methods take the arguments `index`, `media`, `playNow`.  The
`insert` method adds the `media` to the playlist immediately preceeding
the specified `index`.  The `replace` method `insert`s the `media`, then
`remove`s the playlist entry that was previously at that position.

This PR depends on \#151, and probably obsoletes \#102

I though I'd get this PR into the queue even though \#151 has not been
reviewed yet