Copy `site_generate` history to this new branch

I did this to enable us to have the site content in a single repository - the source and the generated HTML. It just seems a bit messy to have 2 repo's for one purpose. All history from the `site_generate` repo has been preserved.

To make working between the `master` and `pelican` branches easier, I added this `make` target:

```make
copy_master: publish
    git checkout master
    cp -R output/* ./
```

From there you can review the changes and commit them to `master`.

The workflow is as follows:
* Switch to `pelican` branch: `git checkout pelican`
* Edit Pelican source.
* `git commit -am "Updated Pelican source"`
* `make copy_master` (now we're back on `master`)
* `git commit -am "Updated site content"`

As always, I leave it to you guys to merge this pull request, if you agree with this strategy.