Refactored code

Changed target framework from 4.0 to 4.5
Expiration:
* Removed dictionary imlementation and imlicit conversions
Paste:
* Moved NewPaste() to Paste class
* Renamed NewPaste() to Create()
* Added XML documentation comments
* Renamed Date to CreateDate
PasteFormat:
* Renamed to Language
Pastebin:
* Added XML documentation comments
* Moved NewPaste() methods to Paste class
* Moved const strings to Utills
PastebinException:
* Added new ctor for nested exceptions
User:
* Renemed Default* properties to Prefered*
* Removed devKey field as it was unnesisary
Utills:
* Added const strings from Pastebin
* Added exception handling in PostRequest()