Adding authentication for Subversion

This builds off work started in
PR 838 (https://github.com/capistrano/capistrano/pull/838) which added
basic Subversion support.  There were no options for authentication, which
is used in most Subversion servers (usually when used over HTTP/S).

This adds two configuration variables:

* :scm_username - Username
* :scm_password - Password

When both are set, authentication is passed to the relevant Subversion
commands.