Add indefinite article methods to Inflector

I have added indefinite_article and with_indefinite_article
to ActiveSupport::Inflector and wrapper methods to core_ext for String

This allows the following:

    "elephant".indefinite_article
    # => "an"
    "x-ray".with_indefinite_article
    # => "an x-ray"
    "cat".with_indefinite_article
    # => "a cat"

The logic is a port of the Lingua::EN::Inflect perl module