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.
Merge pull request #1402 from williambogaiv/razorEncodedStringHelper

Add Razor class to HTML-encode strings.