* Added ErrPolicyNesting which is returned when nesting of policies has occured
* Replaces ErrMalformedPolicy in the case of nesting
* Changed test case in bucket-policy-parser_test.go (ErrMalformedPolicy -> ErrPolicyNesting)
S3 API returns BucketAlreadyExists error when some another user has such bucket.
If user that creates the bucket already has it, s3 returns BucketAlreadyOwnedByYou.
As minio has only one user, it should behave accordingly.
Otherwise it causes failures in the applications that ignore creation of already existing bucket in the account, but fail when bucket name is used by someone else.
This API takes input XML input in following form.
```
<?xml version="1.0" encoding="UTF-8"?>
<Delete>
<Quiet>true</Quiet>
<Object>
<Key>Key</Key>
</Object>
<Object>
<Key>Key</Key>
</Object>
...
</Delete>
```
and responds the list of successful deletes, list of errors
for all the deleted objects.
```
<?xml version="1.0" encoding="UTF-8"?>
<DeleteResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
<Deleted>
<Key>sample1.txt</Key>
</Deleted>
<Error>
<Key>sample2.txt</Key>
<Code>AccessDenied</Code>
<Message>Access Denied</Message>
</Error>
</DeleteResult>
```
Fail createObject() if a file already exists and one attempts
to create a prefix/directory by same name.
Send an approriate error back to the client as 409 Conflict.
