moved rss to xml, create xml link extractor and define rss link
extractor as subclass of xml one
added csv and regex link extractors, added tests for link extractors
allow html link extractor to accept also a html response as argument,
for api compatibility with rest of link extractors
csv reader does not accept parameters in unicode
added support in spider for feed start urls, added test, updated project
specs reference and schemas.json
change column type to csv_column, added rss type shortcut.
removed bad docstring
added all the format parameters that supports python csv reader.
moved feed_start_urls as new 'start' request type in init_requests
move initialization line to __init__ (wrongly moved previously inside method)
improve start urls explanation text
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