Moved the repl to a new package

ethereum/cmd.go

ethereum/flags.go

ethereum/main.go

ethereum/repl/javascript_runtime.go

ethereum/repl/js_lib.go

ethereum/repl/repl.go

ethereum/repl/repl_darwin.go

ethereum/repl/repl_linux.go

ethereum/repl/repl_windows.go

ethereum/repl/types.go