Add model article

[bartaelterman]

An article should have the following fields:

• url (text)
• title (text)
• text (text)
• published_at (datetime)

[niconoe]

Just implemented it Please note:

• I took the freedom to use URLField instead a simple CharField for the URL.
• You'll have to run migrations
• I also added the model to the Admin, always useful for debugging!