Implement an LDAP based authentication for the WebStatus #138

Hello!

Here is [one of patches](http://trac.buildbot.net/attachment/ticket/138/0012-Implement-an-LDAP-based-authentication-for-the-WebSt.patch) for the feature [#138](http://trac.buildbot.net/ticket/138), rebased upon the newest `master` branch. I've tried to provide no changes to the original patch, but I had to delete changes to `NEWS` and `docs/buildbot.texinfo` files because they don't exist no more.

Speaking shortly, _it works_. I can connect to my work LDAP server using this authenticator. But this code actually has no unit- or acceptance- tests. If these tests are required to have this patch merged into master, please let me know.