Renamed wp-content, updated .gitignore for symlink

In hopes of removing some of the questions I had when using this repo for the first time, I renamed the original wp-content directory to wp-content-sample. This should hopefully make it obvious that you have to do something in order to get your dev site up and running. The documentation will reflect this soon.

I also added a line to the .gitignore to ignore a new wp-content directory or symbolic link.