Add ability to specify a block to execute for response

Example:
```
stubRequest(@"GET", @"http://www.example.com/").andDo(^(NSDictionary * __autoreleasing *headers, NSInteger *status, id<LSHTTPBody> __autoreleasing *body) {
    *status = 200;
    *body = @"Response!";
});
```
The block passed to `andDo` will not be executed until the request is made. This is useful for varying the response based on object state that changes by the time the request is made, and assertions can be made inside the block to check the state of the object between the "start loading" and "finish loading" states, e.g. `XCTAssertTrue(myObj.isLoading);`.