Config has location - Tests work

 * Made config a bit closer to what's needed in Apache, so that it now
   takes a 'base_location'. When you use it in Apache that'll be the
   '<Diretory /a/b/c>' the '/a/b/c' part will get put in front of all
   relative urls.
 * Made config's empty string (that it returns by reference) be static.
 * Made tests with a location without a '/' on the end
 * Moved the 'is_relative' out of Rewriter_impl.hpp into the more
   accesible 'utils.hpp' file