Update Dropmark ICON #140

* Add dropmark.ico

via http://dropmark.com/images/favicon.ico
Fix a bug that can't post to Dropmark #140

via http://dropmark.com/support/getting-started/browser-extensions-bookmarklets/
Revise Dropmark Model #140

* Add support for video and conversation type
* Fix a bug that don't show a post error message on logout
* Fix a post error message about empty post
* Rename converters to CONVERTERS
* Rename getLastViewedPage() to getLastViewedPageURL()
* Add checkLogin()
* Remove URL, getPostPage(), getLastViewedId()

Close #140