Add autoloads

I have added autoloads for the main commands, so if the package is
installed from marmalade/melpa, a user can use those commands without
requiring features.

Also `(require 'google-translate-core)` was moved to the
`google-translate-core-ui.el` because this file relies on the core
functions, and autoloads don't work without that require.  It is not
necessary for `google-translate.el`.