* Added stopwords filter including filter documentation

* Checked functions using the stopwords filter if they'd be able to deal with an empty stopwords array and fixed if needed
* Came across potential bug in one of these functions and fixed that (class-admin.php, line 10)
Removed superfluous gitignore
Reverted .gitignore to original
Merge branch 'master' of git://github.com/jdevalk/wordpress-seo into filter-stopwords

Conflicts:
	readme.txt
Merge pull request #84 from jrfnl/filter-stopwords

Adding filter for stopwords