Implemented XML UserInfo parsing

and changed Expiration a little
Changed PasteFormat to be like Expiration

Added Parse and TryParse to PasteFormat and Expiration classes
Fixed UserInfro to use <user> node and not the root in the XML parsing
Added PasteInfo parsed from XML

and changed UserInfo to take XElement insted of string when parsing from
XML
Merged classes

Merged classes PasteInfo into Paste class
Merged classes UserInfo into User class
Fixed formating of code etc...
Updated ToString() for Paste and User
Updated Example