Add IgnoreUnknownColumns to CsvFileDescription

[omederos]

Adds feature discussed in #9.

• Modify article.htm to document the changes
• Add unit test to make sure the new functionality works
• Add ToString() method in TypeFieldInfo class for better debugging

A few important things:

• There was an existing (undocumented) IgnoreMissingColumns field, used mainly for UseFieldIndexForReadingData. It was renamed to IgnoreUnknownColumns, since the purpose of IgnoreMissingColumns is covered by IgnoreUnknownColumns now, to avoid having duplicate fields.
• The entire code probably needs to be refactored at some point in the future, so more unit tests will be required. If we keep adding properties to the CsvFileDescription class, the code might become kind of an spaguetti one.

[mperdeck]

Thanks Oscar, I've accepted your pull request. It is now on Nuget as version 1.5.0. and the article on the CodeProject has been updated as well with your changes.