[Proposal] Added get billing_cycle_anchor parameter to buildPayload method

By default when swapping plans on Stripe the end-of-period date does not change and the customer is billed prorate. So for example:

Plan A (current plan) = $29
Plan B (desired plan) = $49

User signed up on 1 July, 2014 and pays $29. User upgrades on 10 July, 2014 and pays (roughly) $16.50. The end-of-period date remains 1 July, 2014.

I wanted to charge customers the difference and move their end-of-period date. I contacted Stripe and they have a currently non-documented parameter for doing just this billing_cycle_anchor. Now given the same scenario the result would be this:

User signed up on 1 July, 2014 and pays $29. User upgrades on 10 July, 2014 and pays (roughly) $23.50. The end-of-period date is changed to 10 July, 2014.

Usage:

    use BillableTrait;
    protected $billingCycleAnchor = "now";

### Stripe (Internal) Documentation

At creation time, passing a non-now value for billing_cycle_anchor allows you to set up a subscription to have its first "regular" billing date be at a specified point in the future.  In particular (and unlike trial_end) billing_cycle_anchor lets you make a prorated charge for a fraction of the month, and then "snap" the billing cycle to the day that you desire.

So, for instance, if on the 20th of January you did a create subscription request with billing_cycle_anchor equal to February 1st, your customer would immediately be charged for about 1/3 of a month, and then subsequently be charged the full amount on the 1st of each month, going forward.

Additionally, when updating an existing subscription, you can pass billing_cycle_anchor=now (you can pass the string "now", just like for trial_end) to force the billing cycle to "reset" and be anchored "now", the way that it would if you were changing intervals.

Concretely, if you had a subscription that billed on the 1st of the month and then on the 18th of February you made an update request with billing_cycle_anchor=now, the subscription would immediately bill in full for the month of February 18th - March 18th, and then bill on the 18th of the month going forward.