Simplify reuse and add automated tests for "Related genes" feature

[eweitz]

This encapsulates code for the "Related genes" example by moving code for that feature into a "kit" in the Ideogram library.

Kits are a new concept in Ideogram. They are tailored modules that reduce how much code developers need to maintain to recreate specific Ideogram examples. So while kits are part of the library, they are not in the core and will not be reused elsewhere in Ideogram. Kits also make it easier to verify complex use cases with automated tests.

The related genes kit is exposed through the new initRelatedGenes method. Example:

config = {
  organism: 'human',
  chrWidth: 8,
  chrHeight: 90,
  chrLabelSize: 10,
  annotationHeight: 5
}

let ideogram = Ideogram.initRelatedGenes(config);

That's 9 lines to recreate a minimal "Related genes" example -- about 350 lines less than before.

[coveralls]

Coverage decreased (-14.9%) to 75.499% when pulling 8d73110 on abstract-related-genes into 33dfdad on master.