Feature request: Support client-side certificates

[johenning]

Exposing an ampache/subsonic-server to the internet could be a incalculable security risk. Luckily webservers like nginx support authentication via client-side certificates before any request reaches a possibly insecure php-backend.
Android already supports client-side certificates and accessing such a server via chrome works out of the box. Any client-side certificate stored in the credential storage of Android can be used inside chrome to access a server that requires client-side authentification (ssl_verify_client on;).

It would be great if DSub could also use the credential storage to access client-side certificates when necessary. Currently no connection can be established, as if the server was not available at all.

(Possibly related to #60)