tar: fixing extracting files that are dirs 

Fix https://github.com/coreos/rocket/issues/250

The dirs are created using [0755](https://github.com/mcuadros/rocket/compare/coreos:master...master#diff-6b4b95c3fb2a650ac8b9a0c5eabdb8b3R13) as default mode, following the same behaviour from the BSD tar.

BSD has behaviour described here, GNU the behaviour that your are looking for. And both implementations has different default permission. If you wish I can match the GNU tar behaviour.

I just tested, with this small entires:
```go
		{
			contents: "foo",
			header: &tar.Header{
				Name: "deep/folder/foo.txt",
				Size: 3,
			},
		},
		{
			header: &tar.Header{
				Name:     "deep/folder/",
				Typeflag: tar.TypeDir,
				Mode:     int64(0600),
			},
		},
		{
			contents: "bar",
			header: &tar.Header{
				Name: "deep/folder/bar.txt",
				Size: 3,
			},
		},
```

The results are:
```sh
>tar -tvf /var/folders/3b/w795p42x12z9_nxhbt1_hpxc0000gn/T/test-tar043628626
----------  0 0      0           3 Jan  1  1970 deep/folder/foo.txt
drw-------  0 0      0           0 Jan  1  1970 deep/folder/
----------  0 0      0           3 Jan  1  1970 deep/folder/bar.txt

tar -xvzf /var/folders/3b/w795p42x12z9_nxhbt1_hpxc0000gn/T/test-tar043628626
x deep/folder/foo.txt
x deep/folder/
x deep/folder/bar.txt

> ls -lash .                                                    Wed Dec 10 07:19:57 CET 2014
total 0
0 drwxr-xr-x   3 mcuadros  wheel   102B Dec 10 07:19 .
0 drwxrwxrwt  27 root      wheel   918B Dec 10 07:18 ..
0 drwxr-xr-x   3 mcuadros  wheel   102B Dec 10 07:19 deep

> ls -lash deep/                                                Wed Dec 10 07:19:59 CET 2014
total 0
0 drwxr-xr-x  3 mcuadros  wheel   102B Dec 10 07:19 .
0 drwxr-xr-x  3 mcuadros  wheel   102B Dec 10 07:19 ..
0 drwxr-xr-x  4 mcuadros  wheel   136B Dec 10 07:19 folder

ls -lash deep/folder/                                         Wed Dec 10 07:20:10 CET 2014
total 16
0 drwxr-xr-x  4 mcuadros  wheel   136B Dec 10 07:19 .
0 drwxr-xr-x  3 mcuadros  wheel   102B Dec 10 07:19 ..
8 ----------  1 mcuadros  wheel     3B Jan  1  1970 bar.txt
8 ----------  1 mcuadros  wheel     3B Jan  1  1970 foo.txt
```

and GNU tar 
```sh
mcuadros@sd-61444:/tmp/foo$ ls -lash
total 1.8M
4.0K drwxrwxr-x     3 mcuadros mcuadros 4.0K Dec 10 07:23 .
1.8M drwxrwxrwt 24790 root     root     1.7M Dec 10 07:22 ..
4.0K drwxrwxr-x     3 mcuadros mcuadros 4.0K Dec 10 07:23 deep

mcuadros@sd-61444:/tmp/foo$ ls -lash deep/
total 12K
4.0K drwxrwxr-x 3 mcuadros mcuadros 4.0K Dec 10 07:23 .
4.0K drwxrwxr-x 3 mcuadros mcuadros 4.0K Dec 10 07:23 ..
4.0K drw------- 2 mcuadros mcuadros 4.0K Jan  1  1970 folder

mcuadros@sd-61444:/tmp/foo$ ls -lash deep/folder/
ls: cannot access deep/folder/foo.txt: Permission denied
ls: cannot access deep/folder/..: Permission denied
ls: cannot access deep/folder/.: Permission denied
ls: cannot access deep/folder/bar.txt: Permission denied
total 0
? d????????? ? ? ? ?            ? .
? d????????? ? ? ? ?            ? ..
? -????????? ? ? ? ?            ? bar.txt
? -????????? ? ? ? ?            ? foo.txt
````