error on building docs on Ubuntu 12.04
```
python make.py html
Running Sphinx v1.1.3
Exception occurred while building, starting debugger:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/sphinx/cmdline.py", line 188, in main
    warningiserror, tags)
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 102, in __init__
    confoverrides or {}, self.tags)
  File "/usr/lib/python2.7/dist-packages/sphinx/config.py", line 216, in __init__
    exec code in config
  File "pandas_github/doc/source/conf.py", line 73, in <module>
    import pandas
  File "pandas_github/pandas/__init__.py", line 15, in <module>
    raise ImportError('C extensions not built: if you installed already '
ImportError: C extensions not built: if you installed already verify that you are not importing from the source directory
> pandas_github/pandas/__init__.py(15)<module>()
-> raise ImportError('C extensions not built: if you installed already '
(Pdb) 
(Pdb) 

```
