Enrich gene cache: Cache API, slim data, mouse, provenance

[eweitz]

This makes the gene cache prototype (#272, #274) smaller and faster. It speeds up the related genes kit.

• Use gene cache to make some blocking API calls non-blocking, improving performance
• Use web Cache API. Fetch gene cache once per Ideogram version, reuse across page loads without network request.
• Make cache smaller:
  • Use slim Ensembl ID that omits organism prefix and zero-padding.
  • Replace stop coordinate with base pair length.
• Add gene cache for mouse (Mus musculus)
• Add header line to cache TSV file indicating provenance, i.e. where the data came from.

[coveralls]

Coverage increased (+0.03%) to 89.111% when pulling 0c8516a373c5eb744f575b8506748bcd18ee6689 on enrich-gene-cache into f7d93fe30c17a3c915fabd9111926c8bf8ae1e55 on master.