contrib/ami-ycmd-next-error.el: connects ycmd with M-x next-error

Added my hack to contrib/ per suggestion in issue #74.
These should be named `ycmd-next-error-*`, e.g. `ycmd-next-error-button`. I know this leads to awkward names like `ycmd-next-error-next-error`, but that could be fixed by using something like `ycmd-next-error-goto-next`.