Add community field

Adds a community field to people.

The feature can be disabled by setting

```ruby
config.disable_communities = true
```

It is expected that wrapper applications will insert the appropriate list of communities into their database using a database migration or some other method.

## Editing screen 
![screen shot 2014-10-29 at 18 51 22](https://cloud.githubusercontent.com/assets/61065/4832504/c0aee41a-5f9c-11e4-823d-81fc44d6c4ca.png)

## Person show screen
![screen shot 2014-10-29 at 18 51 36](https://cloud.githubusercontent.com/assets/61065/4832503/c0a75a9c-5f9c-11e4-979c-8b6813ec7058.png)

## Search results

clicking on the community name runs a search for the given community name

![screen shot 2014-10-29 at 18 52 14](https://cloud.githubusercontent.com/assets/61065/4832505/c0b082b6-5f9c-11e4-9d9b-5b2c217e68f0.png)