Added a menuitem to allow people to update the timestamp on a post

This is related to the drafts pull request (#8) because I basically thought to myself that a person might make a draft, work on it for a few days and then when it's time to publish it the timestamp is all wrong, and then she would have to go and edit the file name manually and no one should have to do that. So I copied the delete menu item and renamed it to "Update timestamp on a post" using the letter "u". When you have selected a post it makes you confirm your action and then it renames it with the current time.

Again, as with drafts, not sure if this fits the use case of how people are using Radiation, I made this just because it's Sunday and I have nothing better to do, and also I have been hit once where I made a post and then wrote it over the course of a day and had to delete and create a new file to have the right timestamp.