MultiGeocoder accepts a custom provider order

This allows `MultiGeocoder.geocode` to follow a provider order different
from the one set globally via `Geokit::Geocoders.provider_order`.

Example:

    Geokit::Geocoders::MultiGeocoder.geocode "Address, City",
      :provider_order => [:google, :yahoo]
Merge pull request #129 from mfilej/custom-provider-order

MultiGeocoder accepts a custom provider order