Add new -overlay option to overlay files prior to templatizing

[markriggins]

- overlays files onto the container at run-time, allowing for entire directory trees of files
  to be dropped into a container based on ENV settings

  `dockerize -overlay "/tmp/overlays/_common/html:/usr/share/nginx/" \
             -overlay "/tmp/overlays/{{ .Env.DEPLOYMENT_ENV }}/html:/usr/share/nginx/" ...`

Small improvements:
- rename Context to EnvContext and move it into template.go, which is the
only place it is used.
- Add early exit to waitForDependencies() if waitFlag is nil
- string_template_eval utility function added
- apply string_template_eval to -template args so users use ENV vars to select the desired template
at run-time like this:
`dockerize -template '/etc/nginx/sites-available/{{ .Env FLAVOR }}.default.tmpl:/etc/nginx/sites-available/default'

Nginx Example improvements
- FROM nginx:1.9 is MUCH faster than compiling your own nginx
- Add a Makefile for examples/nginx
- Install the locally built image of "dockerize" into nginx, this greatly simplifies testing

[markriggins]

I found this confusing -- I thought it was somehow related to golang.org/x/net/context.Context and the ctx variable. So I renamed it to EnvContext and moved it into templates.go, which is the only place its used