Refine variable classes

[tanjeff]

The variable classes (Gauge32,Octet_String,etc.) should be refined. They have certain restrictions (e.g. Counter32 and Counter64 should be always increasing, never decreasing) and relations (e.g. IpAddress may become derived from Octet_String as it is encoded the as an OCTET STRING). Also, the API of these classes should be re-thought considering the purpose and typical usage of them.

[tanjeff]

The variable classes were redefined while implementing table support. They can now be used without inheriting. However, they should emit a (synchronous) signal on GET requests to add an alternative to reimplementing the get() method. Maybe this might also be useful to handle SET requests.