Fix issue 301 by setting the width and height of offscreen text elements to 0.

This fixes this issue:

     https://github.com/readium/readium-chrome-extension/issues/301

Off-screen text elements don't need to disturb the flow by having non-zero dimensions (they still remain visible by screen readers).

Before:

![before](https://f.cloud.github.com/assets/326335/1405269/9443c1e0-3d22-11e3-9fc0-69d098bd120b.png)

After:

![after](https://f.cloud.github.com/assets/326335/1405270/98f744a0-3d22-11e3-9865-28a33e7193a2.png)