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.

R-URL: https://github.com/nodejs/node/pull/1491


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>