Try to interpret domain as a numeric id first

If the supplied id is a number, do not make a costly remote API call
to list all domains; use the number as is.
Use filter option when resolving domain id from FQDN

When we look for a specific FQDN, specify it as a search filter,
instead of getting all the domains in the account and iterating
over them. This makes a huge difference for accounts with many
domains.
Merge pull request #60 from jpetazzo/try-domain-numeric-id-first

Improve domain-to-id lookup code