Object Subscripting

I've added two methods that enable the use of object subscripting for instances of the class ```UICKeyChainStore```.

**Get**
```
NSString *somethingSensitive = store[kStubKey];
```

**Set**
```
store[kStubKey] = kStubString;
```

For purposes of simplicity and to prevent errors, this notation is limited to the set and retrieval of strings by using strings as keys.