Added a completion option to implement all interface functions on a struct.

Start typing `http.ResponseWriter.` -> select **Implement Interface** from the completion menu :

	//Implementing ResponseWriter·Header
	func (this *struct_name) Header() http.Header {
		
	}

	//Implementing ResponseWriter·Write
	func (this *struct_name) Write([]byte) (int, error) {
		
	}

	//Implementing ResponseWriter·WriteHeader
	func (this *struct_name) WriteHeader(int) {
		
	}

It expends from `//Implementing %s·%s\nfunc (${1:this} ${2:*struct_name}) %s%s {\n\t$0\n}\n`.

I'm sure there's a much cleaner way to implement this but I've been using ST3 for less than 48 hours,
any pointers are greatly appreciated.