Change property from NSString to NSRunLoop

"This should render the module compatible with Swift3.0, as well as improving the Objc type checking."

#147

FLAnimatedImage/FLAnimatedImageView.m

@@ -317,7 +317,7 @@ - (void)startAnimating
     }
 }
 
-- (void)setRunLoopMode:(NSString *)runLoopMode
+- (void)setRunLoopMode:(NSRunLoopMode)runLoopMode
 {
     if (![@[NSDefaultRunLoopMode, NSRunLoopCommonModes] containsObject:runLoopMode]) {
         NSAssert(NO, @"Invalid run loop mode: %@", runLoopMode);
@@ -431,7 +431,7 @@ - (void)displayDidRefresh:(CADisplayLink *)displayLink
     }
 }
 
-+ (NSString *)defaultRunLoopMode
++ (NSRunLoopMode)defaultRunLoopMode
 {
     // Key off `activeProcessorCount` (as opposed to `processorCount`) since the system could shut down cores in certain situations.
     return [NSProcessInfo processInfo].activeProcessorCount > 1 ? NSRunLoopCommonModes : NSDefaultRunLoopMode;

FLAnimatedImage/include/FLAnimatedImageView.h

@@ -31,6 +31,6 @@
 
 // The animation runloop mode. Enables playback during scrolling by allowing timer events (i.e. animation) with NSRunLoopCommonModes.
 // To keep scrolling smooth on single-core devices such as iPhone 3GS/4 and iPod Touch 4th gen, the default run loop mode is NSDefaultRunLoopMode. Otherwise, the default is NSDefaultRunLoopMode.
-@property (nonatomic, copy) NSString *runLoopMode;
+@property (nonatomic, copy) NSRunLoopMode runLoopMode;
 
 @end