Add promoter and terminator sequence to genetic engine

[#41020919]
Add definitions of nucleotides paths (reverse engineering of Fablevision images...)

[#41020919]

So, we won't have to use <image> tag, but create these elements by
hand. This will allow us to style them using CSS.
Add new nucleotides module, implemented as d3 reusable component

[#41020919]

I've completely refactored previous implementation and based this one
on approach used e.g. in d3.svg.axis() component.
Style nucleotides in various regions (coding, promoter, terminator)

[#41020919]
Major refactoring of genetic renderer (e.g. nucleotides module, viewport)

[#41020919] [#50058101] [#48761765]

Most important changes:
 - general cleanup (still some work to do),
 - new nucleotides module used,
 - viewport model options are always updated if genetic renderer
   modifies them.
Add viewport slider to "DNA Animations Example"

[#41020919]

So it's possible to take a look at promoter and terminator regions.