Add setting to allow disabling of enumeration validation.

[rakker91]

Sometimes providers change their schemas, adding new enum values not currently in our schema. When that happens, we don't want things to start failing.

By default, QuickFix will blow up if an enum value doesn't match and there's no way to turn this off.

This PR adds the ability to disable this validation, if desired. The default behavior is unchanged.

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[gbirchmeier]

I think this very thing was included in #543. I rejected that part, then the submitters "appealed" to me, then I said that I'd consider it, then... I forgot about it. (Sorry guys!)

I will take another look later this week.

[ubermanl]

Hi all, i just had this issue in production. The market doesn't provide the full spec for all the enum values. As i've seen and gathered from the Java version of quickfix, it's handled by an attribute called "allowOtherValues" included in the enum itself as i mentioned in the followeing ticket #658. I personally like this approach since its more granular and doesnt completely override checkups for all enums.

[VAllens]

Good idea, when to merge it?

[kirsan31]

+1 for this. It's a real problem when provider decide to add some new value to some new enums...
/cc @gbirchmeier