fix #1035 do scraping tests on mock data

don’t store scraped pages with licensed lyrics in repo
nosetests: don't run slow tests by default
Move script to download pages out of tests_lyrics.py

By default (as runned by CI tools), only *fake* example.com page is present in
rsr/lyrics and tests that check content of pages coming from *real* sources are
thus skipped.
Execute lyrics_download_samples.py to download pages from *real* sources. When
done and *real* pages are present on disk, no tests are skipped.
Move script to download pages out of tests_lyrics.py

By default (as runned by CI tools), only *fake* example.com page is present in
rsr/lyrics and tests that check content of pages coming from *real* sources are
thus skipped.
Execute lyrics_download_samples.py to download pages from *real* sources. When
done and *real* pages are present on disk, no tests are skipped.
fetchart: add 'sources' option

Fasten cover search by enabling engines selection and prioritisation.
remove comment
fetchart: import itunes only if necessary
fetchart: make test_art.py tests pass
fetchart: better document 'sources' option
fetchart docs: fix indent
fetchart: fix flake8
docs fetch art: json-like syntax is not mandatory
fetchart: cover engines yield their results
fetchart: fix tests to handle generators
fetchart: fix docs typo
lyrics: remove full texts from lyricstext.yaml

lyrics are considered valid if they contain a set of
selected keywords picked along the original full text.
Merge pull request #1069 from Kraymer/1035_no_copyright

Remove licensed lyrics from tests resources
Merge pull request #1075 from Kraymer/1066-fetchart_sources

fetchart: add 'sources' option
Changelog, tweaks for fetchart sources (#1075)