SNMP v1 and v2c Client implementation

SNMP v1 and v2c client implementation, together with an ASN.1 encoder / decoder (in Foundation) needed by it.
Sample and test cases included, all code fully commented.

There is a raw version that returns the raw ASN.1 objects, and an user-friendly version.

Comments on the design of the classes are welcome.

Sample code:

```
		Poco::Net::SNMPClient snmpcli;

		snmpcli.snmpReply += Poco::FunctionDelegate<Poco::Net::SNMPEventArgs>(tpreply2);
		snmpcli.snmpError += Poco::FunctionDelegate<Poco::Net::SNMPEventArgs>(tperror2);

		snmpcli.get("172.18.1.164", "1.3.6.1.2.1.1.1.0");
```