Merge: Website generation

Hi @nicholsn,

This pull request:

  - Add a `READERS` parameter in `pelicanconf.py` (as described [here](https://github.com/getpelican/pelican/issues/1157)) in order to avoid warnings when calling `fab serve`:
```
WARNING: Could not process specs/xxx.html
'NoneType' object has no attribute 'lower'
```
  - Update `.gitignore` so that files under `content` in the `source` branch are not pushed to the server.

Could you let me know if you are okay with those changes?