Fixed issue that the rendering quality is bad while page curling in iPhone 6s plus.

A new class is added to SkyEpub SDK.

SkyKeyManager
; SkyKeyManager manages the keys for the encrypted epub which is created by SkyDRM solution.

// Creator
-(id)initWithClientId:(NSString*)clientId clientSecret:(NSString*)clientSecret;

// NSString clientId : id to access SkyDRM server.
// NSString clientSecret : secret code to access SkyDRM server.

// gets keys for the encrypted content from SkyDRM Server.
-(NSString*)getKey:(NSString*)uuidForEpub uuidForContent:(NSString*)uuidForContent;
// NSString uuidForContent : uuid of a content which is encrypted inside epub.
// NSString uuidForEpub : uuid of epub itself.