Partial fix for referenceStripElement (#333).

This makes the behavior the same as the navigator (passing an id to
`referenceStripElement` to the main constructor). However, the reference strip
is still anchored to the bottom of the viewer, so it doesn't actually change
the appearance at all.

This doesn't fix the problem, really, but I wanted to submit the PR because I
think that fully fixing it will require some relatively extensive refactoring
of the ReferenceStrip class (I'm not familiar enough with the codebase to do
it myself, I don't think). I do think I see the problem though, so here are
some thoughts on what might be necessary.

The problem comes later (the block starting at [line
129](https://github.com/openseadragon/openseadragon/blob/master/src/referencestrip.js#L129)),
when the controls are added. There, the control is added to the *viewer*,
which anchors the reference strip inside the viewer. This means that although
the ref strip is nominally "inside" the element we passed in, it looks like
it's still in the viewer div.

I think what needs to happen is something similar to what happens in the
navigator class. There ([line
68](https://github.com/openseadragon/openseadragon/blob/master/src/navigator.js#L68)),
the anchor options are set in the initial if/else block (depending on whether
or not the user provides the element or not). It might be a relatively
simple fix to move the anchor logic there in the reference strip class, but it
may also necessitate changes in the API. Offhand, it's tough for me to know 
exactly a vertical reference strip should be implemented if using an
external element, for example. There might need to be some additional options
in the root element (or a clarification of the ones already there).

Let me know what you think...I'm happy to help if I can, but JavaScript isn't
my first (or second, or third) language, so it's tough for me sometimes to
grasp the larger picture.