Added http_client configuration option.

* The `Timezone::NetHTTPClient` is a reasonable default for wrapping
  requests to Geonames. It can now be replaced w/ a user configured one
  by specifying the new class in `Timezone::Configure::http_client`.
* `Timezone::Configure::protocol` was added and defaults to `http`.
* Fixed the api limit reached functionality. The code never worked and
  the tests were not actually testing the functionality.

[Fixes #25]