Do no change settings.TEMPLATE_DIRS on the fly anymore, use thread-locals instead

Instead changing ``settings.TEMPLATE_DIRS`` on the fly, what could not
work properly when there is concurrent requests, we use a thread-local
variable to determine the template directory used for mobile requests,
so the template loader ``opps.contrib.mobile.template.Loader`` can be
used instead to define the right templates in each of your project
views.
Test if mobile users get the right templates directory
Merge branch 'master' of https://github.com/opps/opps into HEAD
Merge commit 'HEAD@{3}'
Merge commit 'HEAD@{2}'