Merge pull request #73 from kalessin/feeds

Generalization of link extractors (added classes and allow to define through specs) and added support for feed start urls
depend on generic Scrapy package name with required min version 0.16
Revert "depend on generic Scrapy package name with required min version 0.16"

This reverts commit 2116a590d0d701d09df1d590153454b840b268d9.
add explicit dependency on scrapy 0.18 and 0.19