plugins cleanup, update descriptions

plugins/digest/digest.txt

plugins/flattr/flattr.php

plugins/googleplus/googleplus.php

plugins/identica/identica.php

plugins/import_export/README.txt

plugins/import_export/import_export.php

plugins/instances/README.txt

plugins/mail/README.txt

plugins/mail/mail.php

plugins/note/README.txt

plugins/pinterest/pinterest.php

plugins/pocket/pocket.php

plugins/redditimgur/README.txt

plugins/share/README.txt

plugins/updater/README.txt