Update deliciousapi.py

rewrite all the parse rule using lxml.
rewrite the get_bookmarks function to get user's data smartly,if the user have more than 100 bookmarks,it will parse html to get data,other it will get json data back from feeds.
if we can satisfy our need using json, we needn't download the whole page then parse it.
so the mechanism can save bandwidth and be more efficient.