Added IsPrimitiveIntegerType
Added EnumHelper

- Added HasFlag (new in .NET 4.0)
- Moved Debug and Release to .NET 2.0 by default
Marked for FAT builds only
Added comment "Added in .NET 4.0"
Added Stream.CopyTo and IEnumerable.Zip

The methods Stream.CopyTo and Enumerable.Zip where added in .NET 4.0
Added comment on ToComparer

The extension method Comparison<T>.ToComparer is the replacement for
Comparer.Create(Comparison<T>) that was added in .NET 4.5
Added ReadOnlyDictionary
Mention to ReadOnlyDictionary in readme

- Added mention to System.Collections.ObjectModel.ReadOnlyDictionary
- Ordered feature list alphabetically