Implement themes (issue #19)

Define theme (for now) using define.ini
Modify functions.php load() and template to adjust for themes directory if defined.
Load stylesheets differently by adding theme directory into href of all styesheets.
Change relative paths for stylesheets (replaced '../' with '../../')
Define THEME as 'default' in index.php if not already defined.
Merge branch 'themes' into dev