Add getChildrenAccountsForAccount method to SBClient class

Added working code for getChildrenAccountsForAccount method in SBClient+Account.m that returns a list of child Accounts for the provided SBAccount object.
I don't fully understand RAC but this is my best guess for what you want:

return [[self rac_GET:[NSString stringWithFormat:@"account/%@/children", account.identifier] parameters:[self requestParametersWithParameters:nil]]
            cb_deserializeWithClient:self modelClass:[SBAccount class] keyPath:@"data.child_accounts"];

There is an error occurring in deserializeModelOfClass:fromJSONDictionary: - any idea what I'm doing wrong here?