Features/issue 124

Patch for https://github.com/mozilla-services/heka/issues/124

Errors are checked by comparing TOML fields against struct attributes.  For any TOML fields that cannot be mapped onto a struct attribute, a secondary map is checked for a white listed list of struct field names:

https://github.com/mozilla-services/heka/blob/features/issue_124/pipeline/config.go#L345

Any TOML field which fails to be in both the white list and on the struct, will trigger a configuration error.

This patch requires https://github.com/bbangert/toml/pull/1 to be merged into the go-toml codebase