refactor godef error handling

split out error checks and error reporting so that other functions can
reuse them.
add godoc-at-point

show godoc for the function/method/type/variable at point. Due to
current limitations in godef and godoc, a method lookup discards the
type, which will result in documentation for both functions and methods
with the same name.
suggest flycheck as well as flymake and fix URL markup
write our own implementation of go--delete-whole-line

Using flet to replace kill-region with delete-region was a hack, flet is
deprecated and at least two people (probably due to a bug in a third
party package) have had their kill ring broken by our hack.
more compact implementation of go--delete-whole-line
style adjustments as per CL 106010043
style adjustment
add documentation for godoc-at-point
simplify godef--successful-p
simplify godoc-at-point
improve README

restructure the README, drop most references to the Go 1.0.3 go-mode as
there's no reason to remind people of all the terrible bugs.
when parsing GOPATH, split on the platform-specific path separator
readd link to gocode in README
import Go's license
remove LEGAL, treat this repository as a mirror of misc/emacs