Users can make payments for their reservations

Woot.

Basic Stripe Setup:
* Add `stripe` and `dotenv-rails` gems
* Create `.env` (and `.env.sample`) w/ stripe API keys
* Assign Stripe API keys from ENV in `config/initializers/stripe.rb`
* Add Stripe info (and database seed info) to README

Models:
* Persist orders in the database and create `Order` class
* Add `stripe_customer` column to `users`
* Add helper methods to `Reservation`

Routes:
* Make `reservations#show` a top-level route
* Nest `orders#create` route under `reservations#show`

Controllers:
* Implement `orders#create`
* Update `reservations#show` so a user can't view a reservation if it's not theirs

Views:
* Add payment form to `reservations#show`
* Extract partial for showing the payment form

https://trello.com/c/ldF3Qy5o

@nnarh @CharlesMassry 

I might replace these lines with:
```ruby
self.update(
  stripe_charge: stripe_charge.id,
  amount_paid: reservation.total_price
)
```