Added extra spacing after 'Sources'.

Increased spacing between 'Source(s)' and photos, yearlist or
the next date.
Renamed setting DEFAULT_FILTER_TAGS to DEFAULT_FILTER_TAG.

The setting contains a single value which is used to select Entry
objects with an EntryTag where the slug matches the setting.
Changed way scrapers are defined to allow any scraper to be used.

Previous versions assumed the scrapers used to download checklists
would always use eBird. To allow other databases, such as Birdlife
International's WorldBirds the template used to define the scrapers
was changes to an array of two-tuples. The first value in the
tuple is the name of the scrapy crawler and the second is an array
of key/value pairs that are passed as arguments to the scraper.