Bug 774234: we can now define per locale l10n blocks

The blocks can be any of these 3 style:

{% l10n whatsnew_feature locales=ru,bn-IN 20120712 %}
{% l10n whatsnew_feature 20120712 %}
{% l10n whatsnew_feature, 20120712 %}

For the First example we will have templates in the given two
languages.
2nd and 3rd example, we will have the templates for all locales.
Bug 774234: Add tests for l10n blocks and langs support.

Refactor template handling and parsing.
Merge pull request #614 from pmclanahan/bug-774234-l10n-block-locales-support

Bug 774234: Add langs definition to l10n blocks.
Merge branch 'master' of git://github.com/mozilla/bedrock