LDAP: ws cleanups, test load ldapjs in register,

###  reworking of PR #636 
* whitespace cleanups
* strict
* jshint
* try loading `ldapjs`
* only register rcpt_to hook if `ldapjs` loads
* corrected syntax of logging calls
* added per-domain LDAP config
* make sure domain defined before using
* move host_list functions from `rcpt_to.in_host_list` to `rcpt_to.host_list_base`
* added tests for `rcpt_to.host_list_base` and `rcpt_to.ldap`
Umm, sure...
That's something I know almost nothing about. Perhaps you'd like to create a PR against this branch? Or just merge this and create another PR that does that?