Assorted changes.

 * Fix duplicated character warning in url regex.
 * Don't crash on null relationship values.
 * underscore_separate multi-word class names for the entity uris.
 * Allow belongs_to? without associated has, will be ignored for
   calculating the full uri for the resource.
   * Still need a way to set which belongs_to? is the primary one in
     cases where there are multiple with inverse has relationships.
 * Use regexes for paths so can correctly match the format, might need
   some modifications to the character classes used.
 * Use #to helper to calculate the root uri and recalculate every
   request so it's always correct.