Serialization subspec can't be used standalone

It refers to `AFNetworkingErrorDomain` and `AFNetworkingOperationFailingURLResponseErrorKey` which aren't defined in the files included by the Serialization subspec.

As a quick fix I moved those two plus `AFNetworkingOperationFailingURLRequestErrorKey` (just to keep them together) into `AFURLConnectionOperation.[hm]`. All other references seem to come from specs that depend on Serialization, so everything's covered.

For what it's worth, `AFNetworkingErrorDomain` doesn't seem to be used outside the Serialization subspec, so it probably belongs there anyway. However `AFNetworkingOperationFailingURLResponseErrorKey` is only used once, so there might be some refactoring possible to move the keys back into the NSURLConnection subspec.

Here's a linker error for reference:
```
Undefined symbols for architecture i386:
  "_AFNetworkingErrorDomain", referenced from:
      -[AFStreamingMultipartFormData appendPartWithFileURL:name:fileName:mimeType:error:] in libPods.a(AFURLRequestSerialization.o)
      -[AFHTTPResponseSerializer validateResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFJSONResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFXMLParserResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFPropertyListResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFImageResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFStreamingMultipartFormData appendPartWithFileURL:name:fileName:mimeType:error:] in libPods.a(AFURLRequestSerialization.o)
      -[AFHTTPResponseSerializer validateResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFJSONResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFXMLParserResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFPropertyListResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
      -[AFImageResponseSerializer responseObjectForResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
  "_AFNetworkingOperationFailingURLResponseErrorKey", referenced from:
      -[AFHTTPResponseSerializer validateResponse:data:error:] in libPods.a(AFURLResponseSerialization.o)
ld: symbol(s) not found for architecture i386
```