findAll: Read partitionKey from options

[jannyHou]

See parent story: #214

Acceptance Criteria

• For method Couchdb2.prototype.all()(which maps to model method find), get the partition key from options like:

   Customer.find({
      where: { name: 'somename' }
   },
   {
      // options
      partitonKey: 'US'
   });

  If partitionKey is found, then invoke driverInst.partitionedFind(partitionKey, query) to find documents.

[jannyHou]

Discussion point: The notation of option:

• {partitionkey: 'US'}
  or
• {cloudantOptions: {partitionKey: 'US'}}

Let's decide which one to use when implement this feature.

[raymondfeng]

It should be {cloudant: {partitionKey: 'US'}} or {partitionKey: 'US'}.

[jannyHou]

Closing since PR merged