Add Life Stages to the Guide Creation Process

An update that adds life stages to the guide creation process. Relevant to #311, waiting on #237 to move forward.

# What's changed
* Now you can choose stages to add to your guide
![screenshot 2014-12-04 11 49 27](https://cloud.githubusercontent.com/assets/485583/5293816/a90b629a-7bab-11e4-989e-5fd0f5b1587c.png)
* And you can choose what shows up in those stages:
![screenshot 2014-12-04 11 50 20](https://cloud.githubusercontent.com/assets/485583/5293820/c0ade8b4-7bab-11e4-9538-9f5aec0de84c.png)

# What's next
* Enabling image uploads for the stages (the frontend part of this is done, just need to know how to actually send them to the backend (#237, @rickcarlino)
* Visual styling of the last page to be in line with the guide creation flow (recruit @warpling).
* Start thinking about how Stage Actions will work.
* Probably remove the requirements model for each guide.
* Add a couple more things to the F.A.Q. Probably think of how the FAQ can be edited on here and read automatically into the site (a faq.md in assets that gets loaded with javascript and converted to the right thing?)
* Guides will need to be visually updated (recruit @warpling again).
* Guide editing will need to remove the instructions field.

I will break those above out to their own issues.
Do you really want to have a field on a model called `where`? Isn't that going to cause a naming collision for database queries with a `where` clause defined by Mongoid?