go: update go build rules

go: update go build rules

Replace go_shared_library with go_mojo_application
that does the same + copies library to root_build_dir.
It makes go applications more convenient to use.
Updating README files with new instructions.
Also changing go echo client to talk to go echo server.

R=jamesr@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/88023de282255d472436de08e8aadcccb7d09cdd

[rogulenko, 2015-04-22 04:42:42]

rogulenko@google.com changed reviewers:
+ jamesr@chromium.org

[jamesr, 2015-04-22 05:21:51]

lgtm

[rogulenko, 2015-04-22 18:43:25]

Committed patchset #1 (id:1) manually as
88023de282255d472436de08e8aadcccb7d09cdd (presubmit successful).