User can Specify Sort Field

* Column headings clickable to sort

* Added to both city & people resources

[#54955710]
Add default sort if none specified

* People defaults to :last_name

* Cities defaults to :name

* Style-fix: replace '=>' with ':' syntax

[#54955710]
Update City sort to include asc/desc toggle

* Based on Railscasts #228, goes one step further, by adding ascending /
  descending on click.

* Protects against SQL injection by restricting the params that can be
  passed to the controller.

* Adds code to application_helper, so that it can be used in other
  resources, i.e. DRY.