(#15) Mention DuckDuckGo when sharing links on Twitter

When sharing a story, page or a search, display “via @duckduckgo for
Android” instead of “via DuckDuckGo for Android” for any app that
contains “twitter” in its package name.

Based on http://stackoverflow.com/a/8550043 and
https://gist.github.com/mediavrog/5625602.
Fixes #15. Second patch.

Made fixes based on comments suggested in previous patch.
Merge pull request #109 from anirudh24seven/anirudh24seven/twitter-share-link-mention-duckduckgo

(#15) Mention DuckDuckGo when sharing links on Twitter