Moved servers to go-whois/servers and core implementation go go-whois/types.
Remove server_ prefix from filenames
types -> core
Collapsed core back into whois. Requires clients to import:
```
import(
  "github.com/domainr/go-whois/whois"
  _ "github.com/domainr/go-whois/servers"
)
```
Not perfect, but not a big deal.
Correct build instructions.