debugger: introduce exec method for debugger

In debugger, the usage of `repl` very ugly. I'd like there is a `p`
like gdb. So the `exec` is coming.

Usage:

```
$ ./iojs debug ~/git/node_research/server.js
< Debugger listening on port 5858
connecting to 127.0.0.1:5858 ... ok
break in /Users/jacksontian/git/node_research/server.js:1
> 1 var http = require('http');
  2
  3 http.createServer(function (req, res) {
debug> exec('process.title')
/Users/jacksontian/git/io.js/out/Release/iojs
debug>
```

And the `repl`:

```
debug> repl
Press Ctrl + C to leave debug repl
> process.title
'/Users/jacksontian/git/io.js/out/Release/iojs'
debug>
(^C again to quit)
```

The enter and leave debug repl is superfluous.

Author: JacksonTian

doc/api/debugger.markdown

@@ -144,6 +144,7 @@ after)
 * `watchers` - List all watchers and their values (automatically listed on each
 breakpoint)
 * `repl` - Open debugger's repl for evaluation in debugging script's context
+* `exec(expr)` - Execute expression in debugging script's context
 
 ### Execution control
 

lib/_debugger.js

@@ -1575,6 +1575,17 @@ Interface.prototype.repl = function() {
   this.repl.displayPrompt();
 };
 
+Interface.prototype.exec = function(code) {
+  var self = this;
+  this.debugEval(code, null, null, function (err, result) {
+    if (err) {
+      self.error(err);
+    } else {
+      self.print(result);
+    }
+  });
+};
+
 
 // Exit debug repl
 Interface.prototype.exitRepl = function() {
@@ -1717,11 +1728,12 @@ Interface.prototype.trySpawn = function(cb) {
     client.connect(port, host);
   }
 
-  self.print('connecting to ' + host + ':' + port + ' ..', true);
   if (isRemote) {
+    self.print('connecting to ' + host + ':' + port + ' ..', true);
     attemptConnect();
   } else {
     this.child.stderr.once('data', function() {
+      self.print('connecting to ' + host + ':' + port + ' ..', true);
       setImmediate(attemptConnect);
     });
   }

test/debugger/test-debugger-repl.js

@@ -49,10 +49,15 @@ addTest('sb("setInterval()", "!(setInterval.flag++)")', [
 
 // Continue
 addTest('c', [
-  /break in node.js:\d+/,
+  /break in timers.js:\d+/,
   /\d/, /\d/, /\d/, /\d/, /\d/
 ]);
 
+// Execute
+addTest('exec("process.title")', [
+  /iojs/
+]);
+
 // REPL and process.env regression
 addTest('repl', [
   /Ctrl/