Update strings.xml

· BEFORE:
    <string name="menu_item_user_messages">Mensajes para "%1s"</string>
(It means something like: Messages to "%1s")


· AFTER:
    <string name="menu_item_user_messages">Mensajes de "%1s"</string>
(It means something like: Messages by "%1s")
Merge pull request #101 from moshpirit/patch-3

menu_item_user_messages