Update tutorials, start table tutorial.

The tutorial for SNMP table support only includes the extended MIB by
now.
Fix wrong path in #include's.
Add new class Oid.

The new class represents an OID. The OidVariable class will be reworked
to use Oid internally for storing the its value. This will separate the
functionality of an simple OID value from the functionality of an SNMP
variable providing an OID value.
Reduce functionality provided by OidVariable.

Some functionality was moved to the new Oid class.
Add Abstract::toOid().

This method was formerly implemented in some subclasses of
AbstractVariable. I added the method to AbstractVariable and
implemented it in all subclasses (where it was missing).
Rework Table and TableEntry to ease API.

The TableEntry class now provides mwthods for returning all variables
and for returning the index variables. The methods for calculating the
index and for returning a specific variable based on its Oid were
removed.
Update tutorials (use Oid instead of OidVariable).