Add Balance Transaction support and tests

This is a new version of #131, updated to merge in other recent changes and remove any breaking properties.  This adds support for Balance Transactions, which are needed to see the fees associated with a charge.  All filtering options listed in the current API documentation are supported.

Something which I included in the previous version but not this one was renaming existing properties called "BalanceTransaction" to "BalanceTransactionId".  This is for consistency, and will be more useful once support for expandable properties is in (I have another update coming with this later).