New Downloader Class, Refactoring, & Moar Javadoc

- Introduced a new abstract Downloader class for custom Downloaders
- Made custom OkHttpDownloader
- Moved Downloaders & Parsers to subpackages
- Main class load() API now uses request objects rather than individual
parameters
- Added more Javadocz
- Instance builder allows for custom Downloader

Hopefully this makes things more flexible as now you can handle all
types of content with custom Parser/Downloader classes. This library is
no longer exclusive to just WordPress blogs. :3