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.
From this endpoint, the real interest is in the `child_accounts` key which is an array of SBAccount-representative JSON data. You should be able to use the built-in CocoaBloc RACSignal helper method for deserializing an array of models, to replace this (effectively NOPing) `map:` block. 

Something like:
```objc
cb_deserializeArrayWithClient:self modelClass:[SBAccount class] keyPath:@"data.child_accounts"
```