Enhanced fatal calls so exit handlers can be invoked

While GO offers the ability to recover from panic there is no way to intercept an os.Exit event. To allow graceful shutdown and clean-up or programs which use Logrus to Fatal out I've borrowed ideas from the `atexit` package and enhanced Logrus.

Usage:
* When setting up the logger one call `RegisterExitHandler( func() {...} )` to add a handler that will be invoked for any `Fatal` call to the logger.

logger.go

@@ -108,7 +108,7 @@ func (logger *Logger) Fatalf(format string, args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatalf(format, args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panicf(format string, args ...interface{}) {
@@ -155,7 +155,7 @@ func (logger *Logger) Fatal(args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatal(args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panic(args ...interface{}) {
@@ -202,7 +202,7 @@ func (logger *Logger) Fatalln(args ...interface{}) {
 	if logger.Level >= FatalLevel {
 		NewEntry(logger).Fatalln(args...)
 	}
-	os.Exit(1)
+	Exit(1)
 }
 
 func (logger *Logger) Panicln(args ...interface{}) {