Usability improvements to OS X MP app.

[ADDED]     OS X: A status item to activate the MP window.
[ADDED]     OS X: A global hotkey (cmd-ctrl-p) to activate the MP
            window.
[ADDED]     OS X: Make the MP window dismissable by hitting Esc.
[ADDED]     OS X: Copy the site content by hitting Enter.
[FIXED]     OS X: Make the password field first responder.
[FIXED]     OS X: Don't pop the password window multiple times if the
            application gets activated while the key isn't set yet.
[IMPROVED]  OS X: Remove the MP icon from the dock.

MasterPassword/Mac/MPAppDelegate.h

@@ -12,6 +12,7 @@
 @interface MPAppDelegate : NSObject <NSApplicationDelegate>
 
 @property (assign) IBOutlet NSWindow                                    *window;
+@property (strong) NSStatusItem                                         *statusItem;
 
 - (IBAction)saveAction:(id)sender;