factor out BITHockeyAppClient to provide network services

* factor out HockeyAppClient
* configure Authenticator with HockeyAppClient
* move appendPostValue to HockeyAppClient (& add the missing boundary parameter)
* move tests over to new file

Classes/BITAuthenticationViewController.h

@@ -9,6 +9,7 @@
 #import <UIKit/UIKit.h>
 @protocol BITAuthenticationViewControllerDelegate;
 @class BITAuthenticator;
+@class BITHockeyAppClient;
 
 @interface BITAuthenticationViewController : UITableViewController
 
@@ -16,6 +17,11 @@
                                requirePassword:(BOOL) requiresPassword
                                       delegate:(id<BITAuthenticationViewControllerDelegate>) delegate;
 
+/**
+ *	must be set
+ */
+@property (nonatomic, strong) BITHockeyAppClient *hockeyAppClient;
+
 /**
  * The application's id to identifiy it in the backend
  */