Metacrawler

Fixed
- search fields and crawlers in instances;

Fixed
- condition for call `clean` method in `Field` instance.

Changed
- not call `clean` method for `Field` instance if value is empty.

Added
- authentication module.

Changed
- `Crawler` and `Handler` now doesn't accept any argument in constructor call - only support inheritance (abstract classes).

Added
- optional argument `data` in `Handler.output` method;

Changed
- `data` attribute in `Handler` is public;

Fixed
- problem with get attribute before call `get_attribute_name` method;

Added
- asynchronous greenlets-based requests for pagination;
- setting any attribute with public method `get_ATTRIBUTE_NAME`;