adding fetched_at attribute
expired? method added
it checks if a page is marked as expired
memory storage added

Useful for testing
Use a dedicated timeout for queue
some comment added
Main core spec added
more test added
a seeded url is always re-downloaded
hack to get created_at if not available

It works on mongo storage only
log added
new example: expired pages
Bump
Merge branch 'incremental_crawling'