Implemented SNMP instance support for custom OIDs

Hey guys, - as promised - here is my very first pull request EVER :)


SNMP symbols can now be defined with an instance prefix:

```xml
<symbol name="ipAdEntIfIndex.192.168.1.200">
    <value>1</value>
</symbol>
```

If the symbol defined using pure ASN.1, the user will be notified
that there is currently no support for that.