Add view option to specify a custom emit value for views

[ellneal]

The current implementation of generated views is to always emit(keys, 1), which allows a simple count to be gathered using the default _sum reduce function. This PR adds the option for the emit value to be specified without having to write the entire map function in javascript.

The example below would create a view that can be queried for total amount taken per client per day.

class Invoice < CouchRest::Model::Base
  property :client_id
  property :date
  property :total_amount

  design do
    view :by_client_id_and_date, emit: :total_amount
  end
end

Generated javascript:

function(doc) {
  if ((doc['type'] == 'Invoice') && (doc['client_id'] != null) && (doc['date'] != null)) {
    emit([doc['client_id'], doc['date']], doc['total_amount']);
  }
}

[samlown]

Very nice!