More TypedCoffeeScript-specific expression

* I changed REPL command prompt from `coffee> ` to `tcoffee> `. `tcoffee> ` shows that we're using TypedCoffeeScript at present, not using original CoffeeScript.

* I changed the leading comment in compiled files from `Generated by CoffeeScript` to `Generated by TypedCoffeeScript`. I think this change clarifies which file is generated by *Typed*CoffeeScript, and helps us to review the source code later.