Updating language specification

[ahejlsberg]

This PR updates the language specification as follows:

• Documents polymorphic this type (Polymorphic 'this' type #4910).
• Documents type parameters as constraints (Type parameters as constraints #5949).
• Fixes Update spec for scoping of interface member names #537, [spec] Parameter properties with question marks and initializers are not clearly specified #541, [spec] Apparent type of string literal type should be global interface String #543, Protected accessibility check does not state that instance members are accessible through derived classes #729, Should not widen in type argument inference #1436, Widening LHS in a dotted property access #1603, Spec integration for ES6 computed properties #1811, Question about change of type relations rules in regards to index signatures #4651, Void type and type parameters constraints #4654.
• Updates terminology from ECMAScript 6 to ECMAScript 2015.
• Updates code generation examples for modules.
• Adds TODO links for all new features.

A good deal of writing still needs to be done to eradicate all of the TODOs, but at least we now have everything collected in one up-to-date document.

[mhegazy]

@ahejlsberg the failing test should be fixed in master now.