Updating the docs regarding deploying to custom domains.

Now that new bosh gems were released, it is no longer necessary to use 'master'.
Also, microbosh's process asks for a new username and password, changed the docs to
reflect that.
clarifying difference between custom and external
minor text change
typo
Merge pull request #65 from cloudfoundry/bosh_domains_doc

Updating the docs regarding deploying to custom domains.