3551: Fix source tree layout.

* Move $GOPATH out of the source tree. ($GOPATH is a workspace. Go
  knows how to import source trees into $GOPATH/src/.) This avoids
  replicating the Go workspace structure inside our source tree, and
  allows Go programs to import our libraries the normal way.

* Add Go SDK documentation page showing how to import.

* Rename keep to keepstore.

* In Python tests, we assume keepproxy and keepstore binaries are
  already installed in $GOPATH/bin/ rather than guessing how you want
  them to be built.

* Delete go.sh wrappers. Dependencies are already installed
  automatically by Go. GOPATH is left alone.
3551: Use keep{store,proxy} from the working dir to build keep containers
instead of running "apt-get install keep".
3551: Make keepproxy tests pass when permission signatures are enabled
in keepstore.
3551: gitignore python/sdk/tests/tmp
3551: Fix more package paths.
3551: When testing keep clients, write keepstore/keepproxy logs to
tmp/run_test_server.log instead of discarding.
3551: When invoking run_test_server.py during tests, pass stderr
through to caller instead of discarding.
3551: Behave correctly when Read() yields some data along with the EOF condition.

"a Reader returning a non-zero number of bytes at the end of the input
stream may return either err == EOF or err == nil."

-- http://golang.org/pkg/io/#Reader
3551: When invoking run_test_server.py during tests, pass stderr
through to caller instead of discarding.
3551: Merge branch 'master' into 3551-go-layout
3551: Handle missing GOPATH and multiple-entry GOPATH sensibly.
3551: Set mode of generated files more aggressively (overrule umask).
3551: Remove chmod 755, no longer necessary now that config.rb does that.
3551: Use FileUtils.rm_r so deleting stale generated/{dir} works.
3551: Remove superfluous chmod(). Umask is 022 here.
3551: Revert "3551: When testing keep clients, write keepstore/keepproxy logs to"

This reverts commit 63871346153fea194b169eec38f6efae3061ebf6.

The default Popen behavior is to use stderr, and that's fine. This was
a mistaken attempt to reveal missing logs, the real solution for which
is 5fcff4c13ae17527ee1efd6fa01d08133212085b.
3551: Merge branch 'master' into 3551-go-layout
Merge branch '3551-go-layout' closes #3551