Fix for handling prefix value and output not formatted according to go fmt.

Use of filepath.Clean on prefix value removes trailing separator. Because of that for command like: 

    $ go-bindata -prefix "/path/to/" /path/to/templates/foo.html

result is
 
```go
go_bindata["/templates/foo.html"] = templates_foo_html
```

instead of

```go
go_bindata["templates/foo.html"] = templates_foo_html
```

Additionally outputs are not formatted "go fmt like": new line characters are missing and byte array is incorrectly formatted.