sebastienros/fluid#578 Implement tag names

[knightmeister]

I've implemented support to retrieve tag names from the statement list as discussed in #578.

ExtensibilityTests have been amended; let me know if I've missed anything obvious.

In terms of design, I opted to add another interface IHasTagName for a few reasons:

1. It means that the various statement types (EmptyBlockStatement, EmptyTagStatement, ParserBlockStatement, ParserTagStatement) now have a shared interface type
2. The above types were all internal and this hasn't changed visibility
3. The root class Statement is used throughout the codebase and wasn't an appropriate change point.

The change means that I can do what I needed to do quite simply, basically

template.Statements.Where(x => x.GetType() == typeof(IHasTagName)).First(y => y.TagName == "what i'm looking for")

I've also added an interface IHasValue for retrieving the interface value from a statement. This Value will hold an individual value passed in, or in the case of using ArgumentsList you'll get an List of FilterArgument.

If you need any amendments let me know.

[knightmeister]

@sebastienros any feedback on this PR or plans to incorporate it?