An initial vignette for Variant Annotation.

@siddharthab I also had to do a bit of the refactoring we discussed to make this work.  

The convert-to-BioConductor objects stuff is quick and dirty for now.  It will get better.
I believe I agree with the comment on that post:
https://www.biostars.org/p/84686/#85902

I'm not sure that insertion example is canonical, and I would be reluctant to code it specially. We don't have actual variant types, so specially handling an insertion seems fragile to me. 

I also really dislike his graphic there - a 0-based numbering scheme isn't actually numbering the spaces between the bases, it just starts at 0 rather than 1 :)

The question of whether an internal is open or closed is actually different from whether we are using 0 or 1 based coordinates - they just happen to be coupled in this instance.