switch to collections

@fardog did some work to use the experimental [collections](http://jekyllrb.com/docs/collections/) feature introduced in the new version of Jekyll (which is supported in github pages too). I think this method will make it a lot easier for people to add themselves.

All you need to do now is add a markdown file to the `_members` directory with some information about yourself in the YAML front matter, e.g.

`_members/jane_public.md`

```
---
# your name
name: Jane Public
# a link to an image you want to use to represent yourself
avatar: https://s.gravatar.com/avatar/532ad97fbf9402eaff6fb6c29ae5faf7?s=64
# a short blurb explaining your life, hopes, and dreams
description: "I do a thing or two"
# a list of skills you'd like to offer to the group
skills:
  - Reading
  - Writing
  - Arithmetic
# some things you're interested in working on or learning
interests:
  - Accessability
  - Social Justice
  - Teaching
# some projects you're involved in
projects:
  - Airbnb for Ducks
# how you'd prefer to be contacted for Code for Portland projects
# (list as few or as many methods as you like)
contact:
  - github: l33th4x0r
  - twitter: catfancy23
  - site: http://your.site
  - email: do-not-reply@your.site
---
```