Geocoder as an interface to allow for chained and cached types

[dougnukem]

Closes #10

Make the top-level Geocoder type an interface and the HTTP specific one a HTTPGeocoder

This will be useful for implementing more abstract Geocoder types like Chained fallback ones or cached ones as described here #10

Also made ResponseObject() be a function that returns new instances (I think there may have been a concurrency issue because the Geocoders were returning the same pointer for every call, so if you made concurrent calls to a Geocoder API it would be decoding to the same response pointer).

// Geocoder can look up (lat, long) by address and address by (lat, long)
type Geocoder interface {
    Geocode(address string) (Location, error)
    ReverseGeocode(lat, lng float64) (string, error)
}

// HTTPGeocoder has EndpointBuilder and ResponseParser
type HTTPGeocoder struct {
    EndpointBuilder
    ResponseParserFactory ResponseParserFactory
}

[coveralls]

Coverage decreased (-0.4%) to 96.476% when pulling d7c5d30 on doug/geocoder_interface into b0ba6eb on master.

[dougnukem]

ptal @codingsince1985 this is a fairly opinionated refactor to turn Geocoder into an interface, but it should clean up things and allow for new possibilities like composing Geocoders to fallback or cached Geocoders.

[dougnukem]

@codingsince1985 we might want to change the coveralls.io "failure" threshold to something like 5% or 10% drop in code coverage instead of reporting any drop as a failure (e.g. when we remove code such that there's less total lines to be covered)

[dougnukem]

@codingsince1985 I rebased my PR's so if you think they're good to go we can merge them in

[coveralls]

Coverage decreased (-0.4%) to 96.476% when pulling 6ce8551 on doug/geocoder_interface into e09cd84 on master.