fetchart (issue 459): use regex to detect valid image names + 'cautious' option for finer control of filename matching
Use re.search instead of re.match with simplified regex.
Add additional cover_names parameter to art_in_path, and makes 'cover_names' a  plugin config option.
Fix unit tests by constructing a FetchArtPlugin in the
fixture and using 'art.jpg' as default image filename so that 
'cautious' option does not interfere with the tests.
fetchart: document 'cautious' and 'cover_names' options
Merge branch 'fetchart-459' of git://github.com/KraYmer/beets

Conflicts:
	docs/plugins/fetchart.rst