Added Aws::S3::Object#upload_file abstraction.

The `#upload_file` method makes it easy to upload a file from disk to S3:

```ruby
# given a S3::Object
obj = Aws::S3::Resource.new.bucket('aws-sdk').object('key')

# upload the file from the given path
obj.upload_file('path/to/source')
```

You can pass a String, Pathname, File, or Tempfile to `#upload_file`. If the file is larger than 15MB, it will be uploaded using the Amazon S3 multipart upload APIs.

```ruby
# upload large files in parts
obj.upload_file('/path/to/large-file')
```