Return prettyprinted JSON strings for errors.

This makes gomega pretty print both the actual and expected strings on an error.  I find that it's easier to see the differences this way.

matchers/match_json_matcher.go

diff --git a/matchers/match_json_matcher.go b/matchers/match_json_matcher.go
index bedf851..efc5e15 100644
--- a/matchers/match_json_matcher.go
+++ b/matchers/match_json_matcher.go
@@ -57,5 +57,5 @@
 		return "", "", err
 	}
 
-	return actualString, expectedString, nil
+	return abuf.String(), ebuf.String(), nil
 }