fix a startup AssertionError, Pygments is missing

``` sh
$ python server.py
Traceback (most recent call last):
  File "server.py", line 9, in <module>
    from selene import options, Selene, web
  File "/home/lowstz/project/selene/selene/__init__.py", line 2, in <module>
    from selene.web.application import Selene
  File "/home/lowstz/project/selene/selene/web/__init__.py", line 4, in <module>
    from selene import helpers
  File "/home/lowstz/project/selene/selene/helpers/__init__.py", line 3, in <module>
    from selene.helpers.string_helper import *
  File "/home/lowstz/project/selene/selene/helpers/string_helper.py", line 79, in <module>
    _bbcode_markup = postmarkup.create()
  File "/home/lowstz/project/selene/selene_env/lib/python2.7/site-packages/postmarkup/parser.py", line 637, in create
    assert pygments_available, "Install Pygments (http://pygments.org/) or call create with use_pygments=False"
AssertionError: Install Pygments (http://pygments.org/) or call create with use_pygments=False
```

I upgrade tornado 2.4 to 3.0, Selene work perfect XD !