Allow the user to customize the stack depth to skip.

[xgfone]

This PR removes the hard-coded stack depth to skip. So the user can customize it by the actual demand.

For example, I want to wrap log15.Logger in my struct.

type MyStruct struct {
	logger *log15.Logger

	// Other fields
	// ...
}

func (m *MyStruct) LogInfo(msg string) {
	m.logger.Info(msg)
}

my := MyStruct{
	logger: log15.New().SetStackDepth(1)
}

my.LogInfo("test")

Before this, I have to discard the stack information of log15.Logger and rewrite it myself.

[notzippy]

Two things missing on this PR

• The SetStackDepth is not declared in the interface, therefore is not exposed
• On the New creation you should set the current StackDepth not use the defaultStackDepth

My additional changes for Revel

[xgfone]

@notzippy Yes, you are right.

I have merged it from Revel