[PATCH] svnlook.py: Make it usable as a library

[[[
* tools/examples/svnlook.py: Make it usable as a library
]]]

Of course it would be better to use iterator interface instead of
copying lists in memory, but I don't know if Subversion bindings have
one. Not all commands are implemented, as I don't have time to
rewrite everything, but at least it is clearly documented what is left,
so it should easy do to if anyone will need it for their hook scripts.

This is the same as:
  https://github.com/apache/subversion/pull/1/files

Please, CC.