misc/emacs: Greatly improve go-mode for Emacs.

The original go-mode is plagued with odd behaviour, lack of
behaviour typical to modes in Emacs and bugs.

This change rewrites great parts of go-mode (basically only
keeping the gofmt and godoc functions).

Additionally it adds new features such as manipulating package
imports.

For more information please see
https://groups.google.com/group/golang-nuts/browse_frm/thread/3a9d6dae3369c0b5/1efe65e2f7afb190

Fixes issue 3618.
Fixes issue 4240.
Fixes issue 4322.
Fixes issue 4671.
Fixes issue 4726.

R=golang-dev, fullung, sameer, cw, arthur, proppy, adonovan, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7314113

Committer: Sameer Ajmani <sameer@golang.org>