Allow integer to be used for enclosure length

This patch allows integer to be used for specifying the enclosure length
instead of just allowing string values. This is certainly the more
natural choice for something representing a number.

With this patch string values will continue to work.

This fixes #104

feedgen/entry.py

@@ -668,7 +668,7 @@ def enclosure(self, url=None, length=None, type=None):
         :returns: Data of the enclosure element.
         '''
         if url is not None:
-            self.link(href=url, rel='enclosure', type=type, length=length)
+            self.link(href=url, rel='enclosure', type=type, length=str(length))
         return self.__rss_enclosure
 
     def ttl(self, ttl=None):