Fix for ampache in subdirectory

The .gitignore change is needed to ignore the customized files in /config/ (motd and registration_agreement).

The htaccess.dist change:
I put my ampache install in http:/site/ampache/. Thumbnails of artwork didn't show up in the HTML5 player and localplay didn't work. A small change to .htaccess fixes it using Apache2's CONTEXT_PREFIX. 

Warning - I don't know how long CONTEXT_PREFIX has been in Apache 2 (at least since 4.2) but it sure is handy. The only doc I've found is http://people.apache.org/~humbedooh/lua_ap/lua_ap.html#apache2.context_prefix