Improve default gitignore

Contents should be as follows.

wp-config.php
.htaccess
robots.txt
wp-content/cache
.DS_Store
.svn
_.bak
*.old
Thumbs.db
wp-content/blogs.dir/
wp-content/upgrade/_
wp-content/advanced-cache.php
wp-content/object-cache.php
wp-content/db.php
wp-content/cache/*
wp-content/w3tc-config/*
