Binary files are not correctly served via the phantomjs_harness

There are two issues here, I guess. One is that the harness does not read the file in binary mode. A fix merely passes 'b' to fs.read when we are serving up a binary file.

The other issue is weirder. When serving a sample png file, the file is changed in some weird way. Specifically:

```
$ ls -l *.png
-rw-r--r--@ 1 applmak x 2669 Apr 4 08:24 test1x1.bad.png
-rw-r--r--@ 1 applmak x 2742 Apr 3 20:12 test1x1.png
```

Even weirder:

```
$ xxd test1x1.bad.png | head -n 3
00000000: efbf bd50 4e47 0d0a 1a0a 0000 000d 4948 ...PNG........IH
00000010: 4452 0000 00ef bfbd 0000 00ef bfbd 0803 DR..............
00000020: 0000 00ef bfbd efbf bdef bfbd efbf bd00 ................
$ xxd test1x1.png | head -n 3
00000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452 .PNG........IHDR
00000010: 0000 0080 0000 0080 0803 0000 00f4 e091 ................
00000020: f900 0002 7350 4c54 45ff ffff 3333 3333 ....sPLTE...3333
```

which seems to indicate some kind of encoding issue? This problem might be an issue with phantomjs itself.