Make scikit-image an optional dependency of photutils

I think there is agreement that `scipy` and `scikit-image` are OK as optional dependencies (see https://github.com/astropy/photutils/issues/38#issuecomment-43100240).

Just in case this needs further discussion or clarification, I'm making this separate PR adding a note to the docs that states this.

cc @larrybradley @astrofrog @eteq @kbarbary @adonath @bsipocz OK with you?