Add support for native subversion clients

Presently Bamboo comes bundled with the svnkit library in to Bamboo which has some limitations compared with the subversion client

Things that are broken in svnkit that work in subversion:

• SSL support in svnkit has bugs, like the TLS protocol bug that triggers the infinite loop bug BSP-9391.
• Svnkit only supported SNI since two weeks ago, and only on Java 7. In contrast, subversion handles SNI out the box.
• Bamboo's use of svnkit forces people to enter the passphrase of the bamboo SSL client certificate on every plan configuration. Native subversion lets you set this up and have all jobs "just work" with no external messing about with authn or authz at all. (To be fair svnkit allows this default bahviour too, Bamboo chooses not to support it and always overrides the svnkit default behaviour).

It would be great to have one similar to the way that Bamboo allows for both jgit and native git clients.