Expose a better interface for automatically forgetting old secrets.

As described in #3, if you have an accessor function `acc`, you can now
perform synchronous operations inside an garbage-collected context using
`acc.autoForget`:

  var acc = require("private").makeAccessor();

  acc.autoForget(function() {
    // Any new secrets created here will be forgotten when this
    // function returns.
  });

Note that you typically don't want to hand out references to the `acc`
function itself, since access to the accessor function is synonymous with
private access, but you can safely hand out references to `acc.autoForget`
without threatening privacy.

For strict memory safety, call `require("private").makeAccessor(true)` to
require that all uses of the accessor function occur within a context
established by `autoForget`.

Fixes #3.
Basic tests of autoForget interface.
Bump NPM version to 0.1.1 in preparation for publishing.