Make global connection options configurable

A HTTP library agnostic connection_options hash strikes me as a better solution than duplication all options provided by the HTTP lib within instagram (proxy, ssl, etc.). First, it decouples the instagram from the HTTP library. Second, it makes it easier for users to set more advanced configurations.

If this pull request is well received, I will send a similar one for request options. In particular, it would be useful to be able to set timeouts (which is done on the request level in Faraday).

Note that the first included commit is just some housekeeping that doesn't change any logic.