fix pagination on global site attributes

Traceback (most recent call last):
  File "/usr/local/bin/wok", line 4, in <module>
    Engine()
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/wok/engine.py", line 97, in __init__
    self.generate_site()
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/wok/engine.py", line 135, in generate_site
    self.render_site()
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/wok/engine.py", line 354, in render_site
    new_pages = p.render(templ_vars)
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/wok/page.py", line 345, in render
    extra_pages = self.paginate()
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/wok/page.py", line 386, in paginate
    source = templ_vars['site']
NameError: global name 'templ_vars' is not defined