Moved "Models" to new project and others

Moved "Models" to new project so both "Client" and "WebAPI" can access
them without having access to each other.
Changed "Post" methods to "ArtistController" and "AlbumController" to
look alike.
Moved Database entity creation in the Models using "Factory"
pattern.
Added "Create" method to "IModel" interface.
Implemented  virtual "Create" method in:
-"AlbumModel";
-"FullAlbumModel";
-"SongModel";
Added "ModelFactory" class.