adding `_scheme` parameter to `url_for`

In order to better facilitate generation of URLs that make use of an HTTPS URL
scheme this patch adds a parameter with this specific purpose in mind. To
achieve this we explicitly pass in a param, `_scheme='https'`, and then set the
`url_scheme` attribute of our `MapAdapter` instance appropriately.

Importantly, `_external=True` must be set in order for this to work properly.
As such, failure to do so results in a `ValueError` being raised.
Merge pull request #667 from maxcountryman/secure-url-for

adding `_scheme` parameter to `url_for`