initial generator functions

[upsampled]

@slayercat do not accept this yet, @jduranf will act as an acceptance tester and submit some additional example code, per #31

Regarding the actual PR, the OID generator function is intended to refresh the values stored in the existing OID structure. #40 should make OID thread-safe now, so the generator function can be called at any time to update values. Currently, the only way for GoSNMPServer to trigger this update is to set OIDsSyncOnWalkStart to true. This tells the code that if a GET-NEXT is received with an OID that is less than any of the previously registered OIDs, it should update the values. It’s a hack, but it works for the typical SNMP walk use case.