closes #816

- 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

- HTTP status or Socket status
- Collect those metric:
    accepted conn:
    listen queue:
    max listen queue:
    listen queue len:
    idle processes:
    active processes:
    total processes:
    max active processes:
    max children reached:
    slow requests:
- Tag metric with: `host` and `pool` name

Closes #255