Add page_type variable for calendar and tag pages

Because undefined local variables throw exceptions instead of returning
nil like instance variables, add a 'page_type' variable to tell the
different types of archive pages apart. Use 'tagname' in place of '@tag'
because a 'tag' local will clash with Padrino's tag helper.
Merge pull request #66 from rumpuslabs/deprecate_instance_vars

Deprecate the use of instance variables in metadata
Remove unnecessary nils and comments.
Merge pull request #68 from rumpuslabs/deprecate_instance_vars

Remove unnecessary nils and comments
Fix broken tests
Merge pull request #69 from rumpuslabs/deprecate_instance_vars

Fix broken tests
Make sure new case extends Article correctly
Fix blog_data again
Create summary from source instead of output

The old way of generating an article summary by splitting at an empty
line could result in unclosed html tags. This patch avoids that for
Markdown source files, by splitting on the source file instead of the
rendered output, and then rendering the summary separately. It also
provides a means to override the default summary generator, if a more
complex algorithm is required.
Merge pull request #70 from rumpuslabs/summary_change

Create summary from source instead of output
Add time zone support in BlogArticle#date
Don't publish post-dated articles
Merge pull request #74 from rumpuslabs/timezone_support

Add timezone support and don't publish future-dated articles
Merge pull request #75 from bhollis/master

Prep version 3.1.0
Add a test for omitting :day from the blog.sources config (from issue #72)
Fix time zone defaulting and allow users to set timezone with 'set :time_zone'
Merge pull request #76 from bhollis/timezone

Fix time zone defaulting
Default templates use correct articles, fixes #77

Calendar and Tag pages set the `articles` local but `page_articles` was
ignoring this when `blog.paginate = false`.
Merge pull request #78 from rumpuslabs/paginate_false_fix

Default templates use correct articles, fixes #77
Version 3.1.1