Avoid expensive fmt.Fprintf calls in `StringWriter.Write` inner loop.

This addresses issue 14.

stringwriter.go

@@ -5,10 +5,11 @@
 package bindata
 
 import (
-	"fmt"
 	"io"
 )
 
+const lowerHex = "0123456789abcdef"
+
 type StringWriter struct {
 	io.Writer
 	c int
@@ -19,8 +20,13 @@ func (w *StringWriter) Write(p []byte) (n int, err error) {
 		return
 	}
 
-	for n = range p {
-		fmt.Fprintf(w.Writer, "\\x%02x", p[n])
+	buf := []byte(`\x00`)
+	var b byte
+
+	for n, b = range p {
+		buf[2] = lowerHex[b/16]
+		buf[3] = lowerHex[b%16]
+		w.Writer.Write(buf)
 		w.c++
 	}