Add success and errors to CBTransaction

Added success and errors to CBTransaction in order to determine if the transaction succeeded.
If success is false, the errors property will be populated with the errors and the other properties
will be nil.
Add support to specify the currency when sending or requesting money

Added support to specify the currency when sending or requesting money.
Use the new send and request methods that have a withCurrency parameter.
Merge pull request #7 from sirnacnud/transactions

Transactions