Handle Octopress Environment

This allows the user to set his or her environment, and for scripts to read them in.

```ruby
require 'octopress'
Octopress.env # reads in the OCTOPRESS_ENV
```

Set `env` in your site.yml:
```yaml
env: development
```

Or, set `OCTOPRESS_ENV` in your shell:

```
OCTOPRESS_ENV=development rake generate
```