Ignore non ascii characters. Bad SNMP Agents might return them.

[fragfutter]

There are some snmp agents around that return latin1 encoded strings.

[vincentbernat]

Triggering an exception was the expected behaviour. There are several tests that check this is the case (and I need to investigate why Travis doesn't work anymore). I want Snimpy to not ignore errors silently. However, I acknowledge that there should be a way to ignore the error.

[coveralls]

Coverage remained the same at ?% when pulling 38f4d1729c39df4cc417e9b37c12c8f40e94ef48 on fragfutter:master into 6968f96f90d144fd9de4ab70d75aec6ceebbea7c on vincentbernat:master.