Rename value_t -> AbstractValue.
Rename Integer -> IntegerValue.
Rename Octet_String -> OctetStringValue.
Rename Opaque -> OpaqueValue.
Rename oid -> OidValue.
Rename Gauge32 -> Gauge32Value.
Rename Counter32 -> Counter32Value.
Rename Counter64 -> Counter64Value.
Rename TimeTicks -> TimeTicksValue.
Rename IpAddress -> IpAddressValue.
Rename variable -> AbstractVariable.
Rename primitive_variable -> Variable.
Rename master_proxy -> MasterProxy.
Improve tutorials and Variable documentation.