`nrows` may change if you restart the server; documented why this is
the case in the design.md file.  It's not a problem.

Passes tests, but doesn't really handle nrows (and removing partially
full files) correctly, when deleting near the end of the data.

Now supports "list /foo/bar" in addition to the older "list --path /foo/bar"

Use numbered subdirectories to avoid having too many files in one dir.
Add appropriate tests.

Also fix an issue where the mmap_open LRU cache could inappropriately
open a file twice because it was using the optional "newsize"
parameter as a key -- now lrucache can be given a slice object that
describes which arguments are important.

Now the goal is 128 MiB files, rather than a specific length.

We replace cStringIO with StringIO subclass that forces UTF-8
encoding, and explicitly convert commandlines to UTF-8 before
shlex.  These changes will only affect tests, not normal commandline
operation.

Spent way too long trying to track down a cryptic error that turned
out to be a CherryPy bug.  Now we catch this using a decorator in the
'extract' and 'intervals' generators that transforms exceptions that
trigger the bugs into one that does not.  fun!

Cleaned up OS-specific path handling in nilmdb, bulkdata.

This will make migrating to my own data storage engine easier.

Warns at runtime if a class's close() method wasn't called before the
object was destroyed.

There's some bug with the testing harness where placing e.g.
  from du import du
in nilmdb/utils/__init__.py doesn't quite work -- sometimes the
module "du" replaces the function "du".  Not exactly sure why;
we work around that by just renaming files so they don't match
the imported names directly.