Changed signatures on constructors

With the latest version of TypeScript some issues were apparent with the constructors for momentStatic returning a type. I looked into it and the correct way to define constructors in interfaces is with the `new` keyword. 

References:
https://typescript.codeplex.com/workitem/925
http://stackoverflow.com/questions/13407036/how-does-typescript-interfaces-with-construct-signatures-work/13408029#13408029