misc/emacs: Add support for godef

godef[1][2] is a third party tool for printing information about
expressions, especially the location of their definition. This can be
used to implement a "jump to definition" function. Unlike
cross-language solutions like ctags, godef does not require an index,
operates on the Go AST instead of symbols and works across packages,
including the standard library.

This patch implements two new public functions: godef-describe (C-c
C-d) and godef-jump (C-d C-j). godef-describe describes the expression
at point, printing its type, and godef-jump jumps to its definition.

[1]: https://code.google.com/p/rog-go/source/browse/exp/cmd/godef/
[2]: go get code.google.com/p/rog-go/exp/cmd/godef

R=adonovan, cw, patrick.allen.higgins, sameer
CC=golang-dev
https://codereview.appspot.com/7781043

Committer: Alan Donovan <adonovan@google.com>