Modify UseCorrectPrefixRule for to allow classes to begin with Is etc.

This allows classes to begin with I<lower><upper> rather than just
I<upper><upper> or I<lower><lower>. This allows for classnames like
InMemoryDoohicky
Merge pull request #20 from krijesta/use-correct-prefix

Modify UseCorrectPrefixRule to support classes begining with two letter words starting with I