Added log adapter for logrus

Also changed standard logger interface to take a map instead of varargs for
extra data.

logger.go

@@ -42,8 +42,8 @@ func (ll LogLevel) String() string {
 
 // Logger is the interface used to get logging from pgx internals.
 type Logger interface {
-	// Log a message at the given level with context key/value pairs
-	Log(level LogLevel, msg string, ctx ...interface{})
+	// Log a message at the given level with data key/value pairs. data may be nil.
+	Log(level LogLevel, msg string, data map[string]interface{})
 }
 
 // LogLevelFromString converts log level string to constant