closing #25 and #18

Adds newlines after blockquotes, allowing for paragraphs after a
blockquote.

Due to merging problems with @lucafrance 's code I had to quickly copy
and paste their code. Thanks for the contribution!

markdownify/__init__.py

@@ -148,7 +148,7 @@ class MarkdownConverter(object):
         if convert_as_inline:
             return text
 
-        return '\n' + line_beginning_re.sub('> ', text) if text else ''
+        return '\n' + (line_beginning_re.sub('> ', text) + '\n\n') if text else ''
 
     def convert_br(self, el, text, convert_as_inline):
         if convert_as_inline: