Use requests library instead of httplib2
Ignore http content-types when finding feed urls

Because of badly behaving web servers http content-type headers might
not be accurate for atom/rss feeds. So we don't trust content-type
headers anymore. Now we try to parse everything as proper feed and if
that fails we can be actually be sure we have something else in our
hands.
Merge pull request #50 from orva/feed-content-types

Feed finding bug fixes