Support for resolving DN based "manager" field about adam

[blaubaer]

This requires also #7 to be solved before.

[feisley]

A few more thoughts on how this might work:

1. An element is defined with type "dn-resolver" to keep things generic
2. The following XML defines the attributes:

<contextAttribute id="manager" >
    <resolvedAttribute id="displayName" />
    <resolvedAttribute id="mail" />
</contextAttribute>

1. In order to resolve the system passes the value of the contextAttribute to .findEntityByDN() and stores the attributes returned in the resolvedAttribute fields. We specify the resolved attributes explicitly to avoid storing the entire resolved record.
2. The attributes can be referenced in Velocity via: $manager.displayName allowing templates such as:

<a href="mailto:$manager.mail">$manager.displayName</a>