SNMPInterfaceCollector inherits from SNMPCollector, but I don't see how it's expected to be importable, since /usr/share/diamond/collectors (on my system) was not installed into any kind of Python-aware namespace. Browsing through some past commits, it seems that SNMPCollector used to live inside of diamond.collector, but it doesn't live there anymore.
As a result, when I start diamond, I get errors when it tries to load SNMPInterfaceCollector, saying it can't import SNMPCollector.
Fixes #14, this fixes the SNMPInterfaceCollector inherit issues and f…
…ixes a few issues in SNMPCollector.