Add a custom user service using an Apache htpasswd file for
authentication.

Add a new class HtpasswdUserService which implements custom
authorization against a text file created with the Apache 'htpasswd'
program.
Moved HtpasswdUserService.java to new source directory according to new
layout after direcory reorganisation since v1.2.1.
Adjusted to Constants.EXTERNAL_ACCOUNT.
Added dependency on commons-codec:1.7