Enable support for Podfiles with use_frameworks!

The previous implementation looked in the main
bundle for the GCDWebUploader resource bundle.
When using the use_frameworks! Podfile option, the
resource bundle will be in a framework bundle, not
in the main bundle.

GCDWebUploader/GCDWebUploader.m

@@ -296,7 +296,7 @@
 
 - (instancetype)initWithUploadDirectory:(NSString*)path {
   if ((self = [super init])) {
-    NSBundle* siteBundle = [NSBundle bundleWithPath:[[NSBundle mainBundle] pathForResource:@"GCDWebUploader" ofType:@"bundle"]];
+    NSBundle* siteBundle = [NSBundle bundleWithPath:[[NSBundle bundleForClass:[GCDWebUploader class]] pathForResource:@"GCDWebUploader" ofType:@"bundle"]];
     if (siteBundle == nil) {
       return nil;
     }