Load templates from Git

This is the beginning of a re-thinking of how we handle templates.

Step 1, decide on a way of loading remote templates without installing them.
Step 2, move all templates except our default into repos or abandon them in favor of community repos
Step 3, Profit!

This is the beginning of Step 1. This PR adds a `middleman git` command which copies the contents of a Github repo into a tempdir using `rugged` and then hands that tempdir over to Thor to generate a new project. This remote clone happens every time the command is run.

An example:

```
middleman git middleman/middleman-templates-default ~/Desktop/test-project
```

No cache. No gems. And for now, no way to specify non-github repos or non-master branches.

This is mostly a conversation starter. I invision this replacing `middleman init` rather than ever shipping as `middleman git`.

I was worried about bootstrapping a project offline, but given that we immediately call `bundle install`, I don't think that actually matters.

Thoughts?