HTML for tweet

fixes #224 

@trevoriancox does this look good to you?

Also, @ryanmcgrath In the files changed, you'll see the function. I am returning specific classes `class=twython-hashtag`, etc.. in case they want to style it different or do something with javascript to it. Should I leave it like this or should I make use of the `mention_html`, `hashtag_html` and `user_html` as variables I have and let them pass their own string formatted custom html or is that to complicated? My vote is for the first and just add the twython custom classes to the anchor tags

Example:

```python
from twython import Twython

t = Twython()
tweet = {u'contributors': None, u'truncated': False, u'text': u'http://t.co/FCmXyI6VHd is a #cool site, lol! @mikehelmick should #checkitout. If you can! #thanks Love, @__twython__ https://t.co/67pwRvY6z9', u'in_reply_to_status_id': None, u'id': 349683012054683648, u'favorite_count': 0, u'source': u'web', u'retweeted': False, u'coordinates': None, u'entities': {u'symbols': [], u'user_mentions': [{u'id': 29251354, u'indices': [45, 57], u'id_str': u'29251354', u'screen_name': u'mikehelmick', u'name': u'Mike Helmick'}, {u'id': 1431865928, u'indices': [104, 116], u'id_str': u'1431865928', u'screen_name': u'__twython__', u'name': u'Twython'}], u'hashtags': [{u'indices': [28, 33], u'text': u'cool'}, {u'indices': [65, 76], u'text': u'checkitout'}, {u'indices': [90, 97], u'text': u'thanks'}], u'urls': [{u'url': u'http://t.co/FCmXyI6VHd', u'indices': [0, 22], u'expanded_url': u'http://google.com', u'display_url': u'google.com'}, {u'url': u'https://t.co/67pwRvY6z9', u'indices': [117, 140], u'expanded_url': u'https://github.com', u'display_url': u'github.com'}]}, u'in_reply_to_screen_name': None, u'id_str': u'349683012054683648', u'retweet_count': 0, u'in_reply_to_user_id': None, u'favorited': False, u'user': {u'follow_request_sent': False, u'profile_use_background_image': True, u'default_profile_image': True, u'id': 1431865928, u'verified': False, u'profile_text_color': u'333333', u'profile_image_url_https': u'https://si0.twimg.com/sticky/default_profile_images/default_profile_3_normal.png', u'profile_sidebar_fill_color': u'DDEEF6', u'entities': {u'description': {u'urls': []}}, u'followers_count': 1, u'profile_sidebar_border_color': u'C0DEED', u'id_str': u'1431865928', u'profile_background_color': u'3D3D3D', u'listed_count': 0, u'profile_background_image_url_https': u'https://si0.twimg.com/images/themes/theme1/bg.png', u'utc_offset': None, u'statuses_count': 2, u'description': u'', u'friends_count': 1, u'location': u'', u'profile_link_color': u'0084B4', u'profile_image_url': u'http://a0.twimg.com/sticky/default_profile_images/default_profile_3_normal.png', u'following': False, u'geo_enabled': False, u'profile_background_image_url': u'http://a0.twimg.com/images/themes/theme1/bg.png', u'screen_name': u'__twython__', u'lang': u'en', u'profile_background_tile': False, u'favourites_count': 0, u'name': u'Twython', u'notifications': False, u'url': None, u'created_at': u'Thu May 16 01:11:09 +0000 2013', u'contributors_enabled': False, u'time_zone': None, u'protected': False, u'default_profile': False, u'is_translator': False}, u'geo': None, u'in_reply_to_user_id_str': None, u'possibly_sensitive': False, u'lang': u'en', u'created_at': u'Wed Jun 26 00:18:21 +0000 2013', u'in_reply_to_status_id_str': None, u'place': None}

print Twython.html_for_tweet(tweet)
```

Which produces:
```html
<a href="http://t.co/FCmXyI6VHd" class="twython-url">google.com</a> is a <a href="https://twitter.com/search?q=%23cool" class="twython-hashtag">#cool</a> site, lol! <a href="https://twitter.com/mikehelmick" class="twython-mention">@mikehelmick</a> should <a href="https://twitter.com/search?q=%23checkitout" class="twython-hashtag">#checkitout</a>. If you can! <a href="https://twitter.com/search?q=%23thanks" class="twython-hashtag">#thanks</a> Love, <a href="https://twitter.com/__twython__" class="twython-mention">@__twython__</a> <a href="https://t.co/67pwRvY6z9" class="twython-url">github.com</a>
```