gclient_scm: use a map for deciding whether or not the scm is supported

gclient_scm: add a checks for unsupported scm and unimplemented method

git_scm: pull out svn code into SVNWrapper

gclient_scm: create a create_scm factory method

Use a factory method to abstract SCMWrapper creation. This method
will eventually return the correct subclass of SCMWrapper. Initially,
there will only be SVNWrapper.

Review URL: http://codereview.chromium.org/215022

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26823 0039d316-1c4b-4281-b951-d872f2087c98