Centralizes https configuration and force TLS only

A lot of code used 'package require http'. This change removed these
requirements and perform once the inclusion in Core.tcl.

For https:// support, ::http::register is called after tls has been
initialized to disable default SSLv3 support (and SSLv2) and enable
TLSv1. That will allow modern post-POODLE SSL negotation compliance.

See http://docs.activestate.com/activetcl/8.6/tls/tls.html