* client: Avoid returning metadata with Get()
client-s3.Get() calls Stat() method because Get() is supposed to return
object metadata. This PR makes a change in client interface so metadata
is returned in Stat() method. This is beneficial for us to avoid calling
HEAD method in S3 when run `mc cat`
* cat: Ignore Stat() returned error
In case of cache gateway, HEAD objects returns 404 even though
objects are gettable. This PR ignores Stat() error since its
purpose in case of cat is to check if the download object size
is equal to what the server announced.
Previously it was failing like below.
```
$ mc ls --incomplete myminio/mybucket
[2016-09-16 09:48:57 PDT] 576MiB 1.iso
$ mc ls --incomplete myminio/mybucket/1.iso
mc: <ERROR> Unable to initialize target ‘myminio/mybucket/1.iso’. Object key is missing, object key cannot be empty
```
This patch fixes this issue.
