phpfpm plugin: enhance socket gathering and config

- If we detect errors when gathering stat via socket, return those error
  so it canbe appear in Telegraf log
- Improve fcgi client, also upgrade it to current version of Go at
  https://golang.org/src/net/http/fcgi/fcgi.go
- Add test for unix socket and fcgi
  to remotely connect but only as an extra url field.
- Allow customization of fpm status path
- Document about using of `host` in case `unixsocket` that it isn't used
- Documet upgrade for new data layout

closes #499
closes #502
closes #538