Logging and profiling support

[klimov-paul]

Should be provided logging and profiling for the sending HTTP requests.
Both features should be configurable, allowing to be disabled in order to save performance.

[klimov-paul]

One thing in particular to think about: should default logging and profiling settings be bound to YII_DEBUG contant?

class Client ....
{
    public $enableProfiling = Yii_DEBUG;
}

In this way profilling will be automatically enabled in debug mode and disabled in production mode.

[klimov-paul]

Resolved by commit 04a4d47