Refactored tests, gemspec and Rakefile.

Simplified Rakefile by removing unused tasks, and leaving only test
task. Cleared out tests and made a speed up by removing setup step which
was generating feed which was unused.

Also cleaned up the gemspec file.
Change SimpleRSS into module, and split it into separate files.

Try to keep most of the original API like `parse` and `new` which are
now calls to the parser. Change class variables which where holding
feed and item tags into constants - you can pass additional tags as
options i.e. SimpleRSS::Parser.new(source, item_tags: [:new_tag])
instead of changing original ones.
Improved utf-8 support between different ruby versions.

Added some assertions against utf-8 strings. Set encoding on top of
parser - if it's ruby >= 1.9 it has __ENCODING__ global variable and it
has already a utf-8 support, rubies < 1.9 doesn't have it and it's good
then to set the $KCODE variable.

Also don't change the string itself when forcing encoding to binary, use
a copy so we don't need to change it back to utf-8. And use it only in
rubies that have force_encoding support.
Use spaces instead of tabs, also run xmllint against test data.
Merge branch 'feature/improve_gem_layout' into stage