Improved attribute name consistency and documentation

[Brian Muse]

I noticed some confusing inconsistencies with some of the models.

Invoice::$allowed uses camel casing for attributes.
LineItem::$allowed however, uses snake case with underscores.

It'd be easier to work with these objects if there was a common style.

On a second related note, it would be hugely beneficial when working with these classes if the properties were documented using PHPDoc in a way that most IDEs support.

For example:

/**
 * @property string $id
 * @property string $subscription_id
 * @property int $date_from
 * ...
 */
class ChargeBee_InvoiceLineItem extends ChargeBee_Model
{

[cb-rakesh]

This has been fixed in v3.0.0