Describe as "offset in document" instead of "viewport"

This module's current description of "offset in viewport" is not intuitive to me and I believe it is an incorrect description.

The position returned is relative to the document, not the viewport. The viewport is the area visible to the user (i.e., the browser window, see [1]), so it would be expected the return value to changes as the user scrolls the document. This is *not* the case for the value returned by this module. I think "offset in *document*" is a more intuitive description.

If you accept this change, I would also update the GitHub repository description and consider publishing to npm as something more descriptive, like *offset-in-document*, instead of *timoxley-offset*.

[1] http://www.quirksmode.org/mobile/viewports.html#link9