* Added new Unit Tests to IMissingLettersTests, most importantly a Performance test using a large input string. Also added tests around case sensitivity comparisons.
* Added input validation to MissingLettersHashSet
* Added MissingLettersHashSetKnockout which is an implementation of IMissingLetters using a Hashset to "Knockout" seen values.
* Added MissingLettersStringContainsToUpper which is an implementation of IMissingLetters that utilizes string.Contains(char) but also makes a call to string.ToUpper() prior to searching
* Added MissingLettersStringContainsEqualityComparer which is similar to MissingLettersStringContainsToUpper however it uses a custom EqualityComparer to make the comparison case-insensitive