Optimise orderBy using indexes

[s-taylor]

Right now orderBy is not using our indexes.
We're using .orderBy('date') in all cases rather than .orderBy({index: 'date'}) when an index exists.
We should optimise our query based on if the field requested to use orderBy is indexed or not (which is now identified in our schema).

https://www.rethinkdb.com/api/javascript/order_by/

[kaievns]

i think the correct syntax is .orderBy(name, {index: 'blah'})

[nwinch]

Resolved by #67