huxley improment

Dear Pete Hunt:
sorry for my poor English first :)

Huxley is  useful and interesting.I do some static html UI test for my project.And i do something improment. Maybe someone need this,so i pull it..

1. I add a test mode "history",
---------------------
in this mode, huxley will save screenshot to a folder named by current datetime after each test.
for exp: this is my 5 tests later
![screenclip](https://f.cloud.github.com/assets/1989073/1113862/c69bde08-1a08-11e3-87af-ddc3efa9bcb2.png)

if you want to use history mode,just record and play in:
```shell
huxley -r -H
huxley -p -H
```

2.i add mouse drag and drop support
---------------
we can drag and drop element now

3.i improve run.py->navigate()
---------------
sometimes, we'll find alert dialog when close or refresh page,i add UnexpectedAlertPresentException to ignore it.

4.i add  huxleytest.py to support nosetests better.
-----------------
Now we can
```shell
huxley -c facebook
```
to create a testcase for nosetests and this case will run in history mode default,it'll log pretty if fails.

5.i add login.py-->login_handle
-------------------------
sometimes,send postdata won't login correctly because of redirct, i provide a login_handle for custom own login method.


That's what i changed.
i'd like that it has something useful. 
I''m a beginner on github, so i didn't create branch for this pull request.sorry..