Make python-beaker an optional dependency
    
    Mako can work fine without beaker.
    Beaker is the preferred caching backend, so the Fedora
    package should pull it in by default, but it should also
    be possible to disable it in more restricted environments.
    
    Notes:
    
    * Beaker is not mentioned as a dependency in setup.py.
    
    * Technically the optional dependency works by having
      imports like the following (from mako/ext/beaker_cache.py):
    
          try:
              from beaker import cache as beaker_cache
          except:
              has_beaker = False
          else:
              has_beaker = True