Support for Balance Transactions

This commit adds a BalanceTransaction entity, and Get/List methods to StripeBalanceService, as requested in #120.  The List method takes an Options class.

The Options class is currently missing the date filtering methods, as I've got a separate Pull Request (#130) which adds support for doing this neatly.  If that is accepted, adding the date filters onto StripeBalanceTransactionListOptions is trivial.