Support settings definition outside of package directory

I wanted to send a pull request for this because I wanted a sanity check on the implementation.

In particular, check out the updated instructions in the README and also tell me if ``OPENELEX_SETTINGS`` makes sense as the environment variable that points to the settings file.  

For example, this is what I added in my ``postactivate`` script:

```
export OPENELEX_SETTINGS=/home/ghing/workspace/openelex-core/settings.py
```