Closes #27.

List of changes:

* Removed Galleries.*, Fetch.Mapping, Fetch.Tree.{CrawlIterator,
fetchIterate, fetchList}.
* Renamed Galleries.List to Crawlers.Linear.Load.
* Added Crawlers module with `Crawler` and `LinearCrawler` classes which
abstract crawler features.
* Removed configuration option from TreeCrawler.
* Added WildcardURL type (=Text).
* Added Crawlers.Examples, which contains 2 functions migrated from
Galleries.Retrieval: pictureList and pictureList'. They were renamed to
fileList and fileList' and turned into Successor functions.
* Prettied up the CLI code a bit.
* Added Helper.String.isNum.