Feature: ability to set date posted and date taken, after photo is uploaded

I've added an API call to change the posted date of the photos after upload, with some configurable parameters.
- Change the posted date to the one retreived from photo EXIF info or to a fixed one (rounded with a granularity parameter)
- Change the taken date to a fixed date and set Flickr granularity (month/year)
Exact parameters are explained in the uploadr.ini.sample