SSH agent support for git and svn integrations

This version allows SSH agent-based authentication to be used for authenticating to SVN or git, as discussed in the ticket http://youtrack.jetbrains.com/issue/IDEA-52034

- Separate out old Trilead SSH2 build213 into private namespace, to avoid conflict with CVS plugin (which depends on a Trilead SSH2 build with patches which were never accepted upstream and do not cleanly merge).
- Import Trilead SSH2 build218 (modern upstream, actively maintained by TMateSoft) for use by non-CVS plugins.
- Import jsch-agentproxy 0.0.6
- Upgrade SVNKit to 885e467 (from the 1.7.x branch)
- Add user interface to SVN plugin allowing agent-based authentication to be selected.