Modified core_user_API.php, login.php and login_page.php to send the user back to the referring page after login.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1036 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9