Move go-specific plugins to ftplugin folder

No filetype plugin should try to do anything during vim's startup (the
plugin folder is sourced on start). The test script was moved into a
folder that vim ignores so to not confuse it's purpose. The oracle
script was moved here as well, as it's not properly setup as a plugin
and should only be sourced when requested.
add guards to silence unnecessary errors

There is already a message that GOPATH isn't set, so let's check for
these bin paths that should be set if GOPATH is set, and silently exit.
Merge pull request #121 from jmcantrell/master

Make vim-go a little more friendly to other plugins and non-go environments