Adds the Tumblr share button
Merge pull request #24 from nitstorm/add_tumblr_share_button

Adds the Tumblr share button
Re-added the migration dependency on the categories app.
Added dependency on python-dateutil to the README.
Converted links in right corner of navbar to dropdown.
Posts are set to DRAFT status by default.
Implemented saving of article contents in the editor.
Added preliminary unit tests (closes #21).
Added Travis CI build image to the README.
Added missing '-i' option for Perl in .travis.yml file.
Corrected issue with SECRET_KEY not being set during test builds.
Added indentation to build commands.
Fixed syntax error in .travis.yml file.
Added 'new article' link to the navbar dropdown.
Added Markdown help page.
Added decorator to allow specific users to view articles.
Added can_edit and can_view methods to the Article model.
Implemented slightly more readable method for protecting unpublished articles.
Fine-tuned the conditions for the can_edit and can_view Article methods.
Added toolbox to the sidebar for articles that allows quick access to the editor.