Charges attributes; new BalanceTransaction; etc.

* Note removal of "count" attribute in recent Stripe update to Lists
* new BalanceTransfer Object (contains Stripe fee) …
* allow passing of "created" hashref to _get_collections,
  used for instance to get Charges constrained by dates
* teach hash_to_object() to convert Stripe's underscore_words
  to module's UpperCamelCase
* additional attributes in Charge object