Made var 'env' available in template rendering and allow subdirectories

To allow for more granular control of the pillar data a couple of changes have been made:

* the fabric env 'env' is now passed in to the jinja template
* when rendering additional templates, cotton now understands the dotted naming scheme and the associated subdirectory structure.

This allows for a top.sls file which looks something like this:

```yaml
base:
  '*':
    - base.common
    - {{env.release_stage}}.common
```

This makes it easier to use standard hierarchical definitions of pillars and the deep merging functionality it provides to minimise changes at the leaves.