Fetch instrument.json and rig.json from SLIMS

[jtyoung84]

• [x] http://aind-metadata-service/instrument/ exists
• [x] route queries SLIMS for an Instrument by its name
• [x] downloads the most recently attached file
• [x] Makes an Instrument model or Rig model depending on SLIMS Instrument type
• [x] render to a response

[mekhlakapoor]

@jtyoung84 @mekhlakapoor Link #175 and #174

[mekhlakapoor]

Closed by #177 and #176