Updated BASIC AUTH support

Added BASIC AUTH support to the current version of iodocs. Support for BASIC AUTH was the subject of these previous pull requests:

https://github.com/mashery/iodocs/pull/73
https://github.com/mashery/iodocs/pull/63
https://github.com/mashery/iodocs/pull/32

and these issues:

https://github.com/mashery/iodocs/issues/157 - the "rejectUnauthorized" flag in app.js in this patch allows self-signed certs. The initial value of this could be moved to a configuration flag instead of always being fale.
https://github.com/mashery/iodocs/issues/10

This change adds support for BASIC AUTH. In apiconfig.json set the auth type to basic
```
"auth":"basicAuth"
```
for any API requiring basic authorization. This will result in the apiKey on the page being replaced with username and password fields.

Note that this pull request does not address using basic auth and apikey at the same time. It would be good if this change could be merged in, then extended to support using both auth types at the same time later.