Instances of NEOCluster were not deleted as soon as the only referrers were
weak proxies (at least that's what a quick check with the 'gc' module showed
at the beginning of tearDown). In some cases, __del__ was called while the next
test was logging a message, which led to deadlocks.

Without those proxies, it may be reliable, but only on CPython. See
  http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies

Relying on __del__ to close a cluster was wrong. NEOCluster is now a context
manager that does it explicitly at exit, in addition to automatically stop it.
The NEOCluster.stop method combines the previous stop/__del__/reset methods.

A new 'with_cluster' decorator is also added to avoid excessive indentation
in tests. Unindentation of existing tests will be done later.