Save post number in ImageLink, for ease of use in GenerateNewFilename

[Issung]

Couple of things:

• Give Tim property some documentation, explain what it is by reading the documentation here: https://github.com/4chan/4chan-API/blob/master/pages/Threads.md
• 1 location still uses the old constructor, remove the old constructor and update the calling location to use the new one.
• Update the now only constructor to take no as a parameter, save it in a new public property (give the new property some good documentation too).
• Rename GenerateNewFilename method to GenerateFilename.
• Rename URL property to Url (don't forget to update the ToString too).