org.httpkit.timer API tweaks

From https://github.com/http-kit/http-kit/issues/11#issuecomment-13113841

This PR is based on 560e471. Let me know if you want me to rebase on something else?

New timer.clj is here: https://github.com/ptaoussanis/http-kit/blob/15303ae1154ae5145b83310cec933983914c9feb/src/org/httpkit/timer.clj