proposal - standardizing directories and template paths

Migrated issue, originally created by jvanasco (@jvanasco)

I've been working on some tools to precompile templates before a webserver forks, and have run into a few issues when trying to build this out more reliably.

1. 'directories' can be duplicates. I'm not sure this will affect anything down the line, but it might make sense. this is relatively simple to fix.

   lookup = TemplateLookup(directories=['templates', 'templates/', 'templates'])
   print lookup.directories
   ['templates', 'templates', 'templates']

2. filenames aren't standardized, so the same template can become 2 objects/compiled 2 ways (relative vs absolute). this will require more thought.

   lookup = TemplateLookup(directories=['templates'])
   t1 = lookup.get_template('goodnight_moon.mako')
   t2 = lookup.get_template('/goodnight_moon.mako')
   print lookup._collection
   {'goodnight_moon.mako': <mako.template.Template object at 0x1015df510>, '/goodnight_moon.mako': <mako.template.Template object at 0x1015df850>}

Start with the TemplateLookup directory handling and the get_template path lookup described in the examples, then inspect how lookup.directories and _collection are populated. Define the expected canonical form for duplicate directories and relative versus absolute template names; done means equivalent inputs no longer create duplicate compiled template objects.