Remove strict dependencies on django, pygments, markdown, jinja2

* addresses Issue #132
* include dependent functionality only if the required package
  is available
* duplicate TemplateDoesNotExist() if django is not present
* add `NotAvailableError` for nodes which depend on non-present
  packages
* move pygments_filter test into template tests
* fix test for Pygments version 1.6
Merge pull request #133 from danring/master

Remove strict dependencies on django, pygments, markdown, jinja2