Separate tags lib with ClojureScript support

As per the discussion on #75, I'm submitting a pull request that splits hiccup into a tag generation library and a compiler library under one repo, and adds ClojureScript support for the former.

The API should be identical to v1.0.5, with a couple of moves and additions resulting from the split:
* `hiccup.core/h` is now a part of `hiccup.util` (it was, after all, an alias for `hiccup.util/escape-html`)
* `hiccup.def/defhtml` now sits under `hiccup.core`
* `hiccup.page/html4`, `hiccup.page/xhtml`, and `hiccup.page/html5` now emit Hiccup data structures instead of strings, while a new namespace called `hiccup.html` has versions of these functions that emit strings

I did not change the version number yet, but I did otherwise update the `project.clj` files, the README, and the Travis config.