Refactored ContextSnippetGenerator

Pattern generation responsibility was moved to `PatternTransformer`.
This allowed to heavily simplify context snippet generator by replacing
`ContextTurnipSnippetGenerator` and `ContextRegexSnippetGenerator` with a
single `ContextSnippetGenerator`.

Refactoring also allowed introduction of additional extension point -
`PatternPolicy`, which allows extension creators to add their own patterns
support to behat (in addition to bundled turnip and regex patterns).

In the userland, `TurnipAcceptingContext` was renamed to `SnippetAcceptingContext`
and `RegexAcceptingContext` was removed in favour of `CustomSnippetAcceptingContext`:

    interface CustomSnippetAcceptingContext extends SnippetAcceptingContext
    {
        public static function getAcceptedSnippetType();
    }

So in order to tell Behat to generate `regex` patterned-snippets for your
context, you do something like that:

    class YourContext implements CustomSnippetAcceptingContext
    {
        public static function getAcceptedSnippetType()
        {
            return 'regex';
        }
    }

This is a bit more verbose setup, but it allows better and cleaner interfaces
with obvious extension point for extension developers.
Fixed locale_configuration.feature broken after snippets generation refactoring