Improved the default-jinja theme, based on the Mako default theme.
Improved base template too.

* Improved base.tmpl to use macros from new base_helper.tmpl
* Fixed namespace collision when post extends base, both of which imported 'helper'
* Backported namespace fix to default mako theme
Converted the site theme to Jinja, using Mako2Jinja, creating a jinja-site theme, which inherits from jinja-default
Merge pull request #469 from dflock/improved-jinja-default-theme

Improved jinja default theme, automatically generated from defult mako one, plus mako2jina.py