File partials

extract inline resources into external partials

Also, fix ERB template interpolation bug (Thor#get method contract for blocks was mis-used)

### Steps to test:
```bash
$ export TEMPLATE_BRANCH=file-partials && \
rm -rf myapp && \
rails new myapp -m https://raw.github.com/thegarage/thegarage-template/file-partials/template.rb
```