Allow for Subversion authentication.

This adds basic support for username/password authentication when using
Subversion, usually over HTTP. It introduces two new configuration variables:

* :scm_username - Username
* :scm_password - Password

Basic Subversion support was added in
PR 838 (https://github.com/capistrano/capistrano/pull/838). This extends that
effort.
Merge branch 'master' of github.com:synewaves/capistrano