Refactor scraper package

[WithoutPants]

This pull request refactors the scraper package to improve (hopefully) clarity, and prepare for further extensibility.

• config type (representing the scraper yml file) has been renamed to Definition and config.go has been renamed to definition.go. Definition is a clearer name for it. The sub-sections have had their type names adjusted as well and their definitions tightened up to only include fields related to them.
• the confusingly named group type has been renamed to definedScraper, to illustrate that it is a scraper derived from a Definition instance. It's constructor has been renamed to scraperFromDefinition, and it's file has been renamed from group.go to defined_scraper.go
• the mapped.go file has been split into multiple files with each file focused on a particular aspect of the mapping functionality
• removed reflection from the mapping code that converts the mapped results into the target object. Type-specific functions with concrete types are used instead. This should hopefully resolve any panics related to incorrect type naming. TODO: handle unknown fields in the mapped results

TODO: integration testing