Features/toml strict

@rafrombrc, @bbangert : I've added a pair of function DecodeStrict and PrimitiveDecodeStrict which throw errors when attributes are found in the TOML which do not map to a struct field.  You can optionally pass in a map of field names which can be safely ignored. 

This patch is part of https://github.com/mozilla-services/heka/issues/124