Rename DefaultPool to Settings

In order to support Vagrant 1.6, we will need a place to store global state.
This patch re-factors the existing `DefaultPool` module into a general
`Settings` module that can fill this role.

Also added YARD docs.
Add validation to the default_pool setting

When assigning a value through `default_pool=`, we now check that it will pass
`IPAddr.new`.

Fixes GH-18.