Add a lyrics backend that scraps results from google custom search api.

Add a 'fallback' option to facilitate working around the 100 queries/day google
limit by marking files as 'visited' so they are not considered for lyrics search
on the next beet run.
I've put my own google_engine_ID as default value in the code but could be
reconsidered, this engine contains databases known to be scrappable by the
plugin algorithm though.
lyrics: restore tags write and fix extract_text()
Code style + fix doc typo
Merge pull request #243 from KraYmer/master

Add a lyrics backend that scrapes results from google custom search api