fix issue #12, broken unit tests in svn 1.7

- use --force for svn add where paths get added twice
- in cases where illegal switches are performed (without --ignore-ancestry)
  just remove the working copy, and check out at the desired switch point.
Merge pull request #13 from OEP/fix-svn-1.7

fix issue #12, broken unit tests in svn 1.7