Add import for build with framework

[ryohey]

Hi Pocket

This pull-request fixes that PocketAPI cannot be built with Cocoapods when use "use_frameworks!"

Podfile

platform :ios, '8.0'
use_frameworks!

target 'pockettest' do
  pod 'PocketAPI'
end

AppDelegate.swift

import PocketAPI

And I got error.

The problem is the order of #import in PocketAPI-umbrella.h.
Cocoapods generates PocketAPI-umbrella.h automatically.

#import <UIKit/UIKit.h>

#import "PocketAPI+NSOperation.h"
#import "PocketAPI.h"
#import "PocketAPIKeychainUtils.h"
#import "PocketAPILogin.h"
#import "PocketAPIOperation.h"
#import "PocketAPITypes.h"

FOUNDATION_EXPORT double PocketAPIVersionNumber;
FOUNDATION_EXPORT const unsigned char PocketAPIVersionString[];

"PocketAPI+NSOperation.h" doesn't include PocketAPI.
I don't know this is the right way to integrate with Cocoapods but this fixes the problem.
Thanks.