snakebin: add `project_type` to zapp.yamls

The default zapp.yaml template now includes a `project_type: python`
directive.
snakebin: add --template arg to zpm new

Even though the (current) implementation of zpm new grabs the python
template by default, it's good to be explicit.
Add python application-specific tutorial

Includes a tutorial on bundling third party dependencies.
Merge pull request #62 from larsbutler/python-apps

Python-specific application tutorial
pythonapps: changed `depends` to `dependencies`

In the initial draft of this feature, the directive was called `depends`
but was later renamed to `dependencies`. I somehow missed this instance
of it when I wrote the docs.
Merge pull request #63 from larsbutler/pythonapps-typo

pythonapps: changed `depends` to `dependencies`