cost estimation

[brollb]

As a user of the app, I would like to get an estimate of the trip fare so I can ascertain that I an not being ripped off.

[sethfri]

We should probably open this given what we talked about regarding moving the cost estimation stuff to the server.

[brollb]

Endpoint will be /mobile/estimate_fare

With GET parameters: distance, duration, origin, destination

[brollb]

And passengers (optional)