core4 redhat url and repo fixes

This branch fixes issues #17 and #19.  It adds an rpm_file attribute and uses it or the user defined rpm_url to set the variable used to name the rpm file.  This is documented in the core4 attributes file.  This branch includes Rob Coward's fix for issue #17 and also replaces the obsolete yum::repoforge recipe with yum-repoforge.   

This branch also fixes a currently undocumented issue when attempting to retrieve files from sourceforge.  The old urls used would result in too many redirects and cause chef to error.  This branch uses the form used in the first redirect from sourceforge, cutting out one redirect. 

All of these fixes are necessary for this cookbook to converge on a rhel or centos 6.4 system.