Multiple headers and footers issue

[zplesac]

We should define how does Mjonir handle cases when we want to add header/footer, but adapter already has defined header and footer.

MjolnirAdapter currently has limitation that it doesn't support multiple headers and views - we can communicate this better to the developer by using one of the few possible solutions:

1. Throw an exception with some meaningful description when we want to add header/footer to an adapter which already has defined header/footer
2. Silently fail (this is current behaviour), but log something to the console
3. Add new addFooter and addHeader methods, which will have an option to force replacing existing header/footer
4. Combine 1 and 3
5. Something different

I would like to first address this issue and later add support for multiple headers/footers, as this will probably introduce some breaking changes.

@kjurkovic @ikocijan @reisub what's your opinion about this?

[zplesac]

Fixed with #9 and release to version 1.1.0.