[ZM] Merge: Remove XML entities during OPAC import

Another opac beautifier function was added to execute a function. The function must have a signature of type `public static java.lang.String functionName(java.lang.String)`. The following example shows how to convert XML entities away from PICA field 021A a (`value` is the full qualified *functionName*):
``` xml
<setvalue tag="021A" subtag="a" value="org.apache.commons.lang.StringEscapeUtils.unescapeXml" mode="function">
    <condition tag="021A" subtag="a" value=".+" mode="matches"/>
</setvalue>
```

**Prerequisite:** Development of this fix needed to be based on #243 *Merge: Allow to map several metadata fields into one (#168) ⅠⅠⅠ*