Add memoization by buffer contents, with tests.
Add info about buffer content memoization to the README file.
Stop memory leaks by having expiring memoization.

The memoization values each have an associated timer, which will
expire it as a certain time since the last update.
Indent to pull it in under 80 columns.
Require ert in tests.
Merge branch 'master' into origin-buffer-memoization

Conflicts:
	memoize-test.el