Add class to HTML-encode strings.
This compliments the existing non-encoded string class.
Move using-statements within namespace declaration.

Also update stored encoded-value type from Lazy<T> to string.
Previous pattern is an outlier (generating the encoded value multiple times from a view-helper) and not really worth the extra baggage.
Changed date serialization
added time element
Whitespace fix
Added setting for turn ISO8601 format on
ISO8601 as default
tidied date serializer test
merge changes from retiainCasing PR
added failing test
increased the constraint score
Added this-prefix
Cleaned up using statements of EncodedHtmlString
Added EncodedHtmlString to string implicit cast
Added missing XML comments