Bug fix for RingBuffer class on Python < 2.7

The RingBuffer class depends on collections.deque's maxlen
property, which was only introduced in Python 2.7. Since we're
already modifying the maxlen behavior (to emit rather than
destroy the evicted element on append), it makes sense to just
implement 100% of maxlen functionality, removing the Python 2.7
dependency.

Also, raise exception if maxlen not set, and raise NotImplemented
on appendLeft(), extend(), and extendLeft(), since those have not
been overriden for proper maxlen behavior.
Bug fix for command line silence limit spec

Fix attempt to modify delta_limits tuple, which is immutable,
when setting explicity peak or IQR limits on the command line,
by making delta_limits a list, not a tuple.
Use __future__ for Python 2.5 context managers

Enable the with statement for Python 2.5. Note that we still can't
support Python 2.5, since the str.format() method isn't backported!

We'd have to use % string interpolation everywhere for 2.5 support.