feat($JPEngine): Add 'po'(print object) and 'bt'(backtrack) command f…

…or debugging in Safari.

'po': print object, inspect the corresponding Objective-C object of JS object  when debugging in Safari;
'bt': backtrack, print call stack of the thread which invoking this JS script.

[USAGE]:

In Safari debubger/console

> po(viewController)
``` output
  "<UIViewController: 0x7fcc75074c00>" = $2
```

> bt()
``` output
0 "0 Demo 0x000000010c1dc88a JPForwardInvocation + 90"
1 "1 CoreFoundation 0x0000000118ecba2e ___forwarding___ + 526"
2 "2 CoreFoundation 0x0000000118ecb798 _CF_forwarding_prep_0 + 120"
3 "3 AlipayWallet 0x000000010c22c003 -[ALPLauncherController setSelectedIndex:] + 291"
4 "4 UIKit 0x000000011c812ada +[UIView(Animation) performWithoutAnimation:] + 90"
5 "5 UIKit 0x000000011c954148 -[UITabBarController _setViewControllers:animated:] + 3761"
6 "6 UIKit 0x000000011c95430b -[UITabBarController setViewControllers:animated:] + 119"
7 "7 Demo 0x000000010aaa78d1 -[UITabBarController(APPageSwitch) alipayPageSwitchSetViewControllers:animated:…"
8 "8 Demo 0x000000010c22536d -[ALPLauncherController initTabBarView] + 1373"
9 "9 Demo 0x000000010c223e16 __29-[ALPLauncherController init]_block_invoke + 38"
10 "10 Demo 0x000000010ad13f69 __34-[SSAuthServiceImpl loginCallback]_block_invoke + 128"
11 "11 Demo 0x000000010a68afa6 APMainCall + 150"
12 "12 Demo 0x000000010ad13eb3 -[SSAuthServiceImpl loginCallback] + 100"
13 "13 Demo 0x000000010ad1188c -[SSAuthServiceImpl tryLoginAsync] + 455"
14 "14 Demo 0x000000010c223b3f -[ALPLauncherController init] + 1215"
15 "15 Demo 0x000000010c231c64 -[LauncherAppDelegate rootControllerInApplication:] + 132"
16 "16 Demo 0x000000010a70ac6a -[DFMicroApplication rootController] + 154"
...
```

JSPatch/JPEngine.m

@@ -138,6 +138,10 @@ + (void)jp_fixMethodSignature
 static NSMutableDictionary *_protocolTypeEncodeDict;
 static NSMutableArray      *_pointersToRelease;
 
+#ifdef DEBUG
+static NSArray *_JSLastCallStack;
+#endif
+
 static void (^_exceptionBlock)(NSString *log) = ^void(NSString *log) {
     NSCAssert(NO, log);
 };
@@ -154,6 +158,17 @@ + (void)startEngine
 
     JSContext *context = [[JSContext alloc] init];
 
+#ifdef DEBUG
+    context[@"po"] = ^JSValue*(JSValue *obj) {
+        id ocObject = formatJSToOC(obj);
+        return [JSValue valueWithObject:[ocObject description] inContext:_context];
+    };
+
+    context[@"bt"] = ^JSValue*() {
+        return [JSValue valueWithObject:_JSLastCallStack inContext:_context];
+    };
+#endif
+
     context[@"_OC_defineClass"] = ^(NSString *classDeclaration, JSValue *instanceMethods, JSValue *classMethods) {
         return defineClass(classDeclaration, instanceMethods, classMethods);
     };
@@ -632,6 +647,9 @@ static void addMethodToProtocol(Protocol* protocol, NSString *selectorName, NSSt
 
 static void JPForwardInvocation(__unsafe_unretained id assignSlf, SEL selector, NSInvocation *invocation)
 {
+#ifdef DEBUG
+    _JSLastCallStack = [NSThread callStackSymbols];
+#endif
     BOOL deallocFlag = NO;
     id slf = assignSlf;
     NSMethodSignature *methodSignature = [invocation methodSignature];