Support bit mask on the RKRequestMethod `method` param

Is there a reason why you didn't use a *bitwise aware* enum that could let us do things like :

```objective-c
RKRoute *createUpdateDeleteRoute = [RKRoute routeWithClass:[Album class]
											   pathPattern:@"/api/album/:albumID/"
													method:(RKRequestMethodGET | RKRequestMethodPUT | RKRequestMethodDELETE)];
```

Just to save us from duplicate lines ;)