Allow user to search for agencies

This allows the user to do either a quick search (just entering a keyword and searching through the agency names, zipcodes, and city) OR doing an advanced search with multiple fields.  Any of the fields can be used but they don't have to be. Additional fields will be added later. 
Consider putting this directly into a model or a presenter. Such as,
```ruby
class Agency

  def full_address
    "#{address}, #{city}, NY #{zipcode}"
  end
end