Fix title-code mismatch, add attribution

[pitkant]

Related to issue #268

• Add attribution to European Commission as in data.europa.eu example (I asked Eurostat used support for clarification whether this is needed)
• Fix mismatches between dataset titles and codes

Still using RefManageR as removing it would need more discussion and possibly deprecation warnings

[pitkant]

I think this PR has become extensive enough and it should be merged to v4-dev. I have documented the changes in NEWS but here are some highlights:

New functions:

• list_eurostat_cache_items()
• get_eurostat_folder()
• toc_count_whitespace()
• toc_determine_hierarchy()
• toc_count_children()
• toc_list_children()
• label_eurostat_vars2() (will replace label_eurostat_vars() shortly)

Old functions with new functionalities:

• get_eurostat_toc() (multilingual support)
• set_eurostat_toc() (multilingual support, make.names = TRUE)
• search_eurostat() (multilingual support, searching codes)
• get_bibentry() (multilingual support, improving internal logic)

[pitkant]

I think instead of reviewing this particular PR it would be more fruitful if you can review and test the whole v4-dev branch