FileResponse: send response either as attachment or inline

This pull requests allows files in response being sent directly into browser instead of forcing them to be downloaded - this is especially helpful when there is a need to show e.g. pdf files directly in the browser window. See related issue https://github.com/nette/nette/issues/1410.

As proposed in the contribution guide http://nette.org/en/contributing, I am adding following information:
- feature
- issues - #1410
- documentation - not needed
- BC break - no

I am really a newbie. I will appreciate if you provide me with some explaining comments in case you find this pull request as not acceptable. Thanks a lot!