Update Hash::Iterator to work on Ruby 2.0

The Iterator was previously using internal details of the st hash
library, but Ruby 2.0 made some significant changes to the structure
that broke our code. I've rewritten the code to use public-API level
code instead and should work fine across all Rubies.

What I changed the logic to is to take the list of keys on the Hash,
which is just a straight Array, which is then trivial to keep an index
"pointer" of the current slot the Iterator is on. Then I let the Proxy
class fetch the value as needed according to the current key.
Fix the require for st.h to work on all Ruby versions
Just assume the rice library exists

The have_library() call was failing now on Ruby 2.0 and Ruby Head, and
looking at it's usage, there's no need for it. The gem won't be
installed unless rice builds completely, so the library will exist.
Error and warn users about missing Ruby shared library on install

Rice doesn't work linking against libruby.a (static) and I'm not sure
how best to make it work. For now to help document this I'm erroring the
installation early if there's no shared library.
Update README on what's supported
Version bump to 1.5
Update annoucement