Adds support for a Scheme property to the Config struct. This enables users
to pass in a URI supported by net/http.HttpClient other than http.

Previously, http was the forced scheme in newRequest() and now http is
defaulted in the DefaultConfig func with the ability to overwrite it
when passing in a Config object to NewClient()
Merge pull request #6 from zerklabs/https-scheme-support

Adds support for a Scheme property to the Config struct