Keyword arguments for lnpostfn (lnprobfn), logp, and logl.

Following up on the suggestion of [Ilya](https://github.com/ipashchenko) in his [comment](https://github.com/dfm/emcee/pull/87#commitcomment-5132212) to push request #87, I have added keyword argument functionality not only to logp and logl in PTSampler, but also to lnpostfn in other classes.