Organize introduction section

Here's my proposed reorganization of the introduction section. I assume we'll have some discussion about it so the commits **have not been rebased**. Once we like what's here I'll rebase and squash.

## Navigation 
The navigation is not broken up into **Introduction** and **Core**. (Question: do we want to change the URLs to reflect this? I left them flat under `introduction`). I like the idea that each of these links displays a small(ish), digestible chunk of information about ROM. I broke out the bit about ActiveRecord and "Why ROM?" as I thought they may be directly linked on occasion.
![introduction 2014-12-14 18-19-02](https://cloud.githubusercontent.com/assets/9002/5430194/bd3aee9c-83bd-11e4-9df6-669ea1141192.png)

## Content
As this pull request is mainly about reorganization I haven't made a full pass on the content. Here's how the broken out content appears now:

#### Introduction `/introduction`
![introduction 2014-12-14 18-24-53](https://cloud.githubusercontent.com/assets/9002/5430210/8a26c890-83be-11e4-9ab5-73e5daf87aea.png)

#### Overview `/introduction/overview`
![overview 2014-12-14 18-25-23](https://cloud.githubusercontent.com/assets/9002/5430213/9b8c37f0-83be-11e4-997b-24ad1ab028dc.png)

#### Why ROM? '/introduction/why`
![why rom 2014-12-14 18-25-57](https://cloud.githubusercontent.com/assets/9002/5430215/ace01d96-83be-11e4-8996-eb5c6723281f.png)

#### ROM and ActiveRecord '/introduction/activerecord`
![rom and activerecord 2014-12-14 18-26-21](https://cloud.githubusercontent.com/assets/9002/5430218/d22fe6e4-83be-11e4-97da-a357ae5dc731.png)

#### Core `/introduction/core`
I added content to `/introduction/core` which pulls the important parts from each core feature page.
![core 2014-12-14 18-22-57](https://cloud.githubusercontent.com/assets/9002/5430207/448e1fcc-83be-11e4-9e78-9b6828f11d75.png)