Add page_helpers and form_helpers from hiccup

[emezeske]

I added support for pretty much everything that hiccup has to offer. The only things I didn't port over were the various html macros, which wouldn't be useful in a ClojureScript context anyway.

I have tested most of the form and page helpers, as well as the more advanced features like with-group and with-base-url, and everything seems to work.

For me, this is a huge win, because I can now do everything in crate as conveniently as I can do it in hiccup.

[emezeske]

Note that I bumped the version to 0.2.0, because this is not backwards-compatible with 0.1.0. The difference is that everything in the crate.tags namespace was moved to crate.form-helpers and crate-page-helpers, to mirror the hiccup layout.