Use home_url(), site_url() and admin_url() where appropriate.

For issue #279

There are many instances of using "siteurl" when home_url() should be used - affects sites where WordPress is installed in subdirectory.

Also we should be using home_url(), site_url() and admin_url() etc rather than get_option( 'siteurl' ).
http://codex.wordpress.org/Function_Reference/home_url#Related