Feature Cache VoterInfo in CoreData

This finalizes our CoreData cache -- there is currently no UI indication that data is stored, and the CoreData cache is wiped on each election query. However, the voterInfo query can be connected to the UI via the following methods in Election+API.h:

```objective-c
- (BOOL) getVoterInfoIfExpired:(void (^) (AFHTTPRequestOperation *operation, NSDictionary *json)) success
                       failure:(void (^) (AFHTTPRequestOperation *operation, NSError *error)) failure;

- (void) getVoterInfo:(void (^) (AFHTTPRequestOperation *operation, NSDictionary *json)) success
              failure:(void (^) (AFHTTPRequestOperation *operation, NSError *error)) failure;
```