Our json implementation did not conform to the spec, and
was missing support for escpaed characters and exponental
numbers. This fixes it, and adds support for reading/writing
json directly from/to a stream.

There are two things left unimplemented. We could use a
"to_json" iface/impl, but that really needs traits to cut
down on code duplication. The other is it wouldn't be that
not that hard to turn this implementation into a event driven
parser like YAJL, but I ran into some type-inference bugs,
so I cut that out. It'd be nice to revisit this in the future
though.

because of the last change, the loop ptr is no longer cleaned up
when the loop exits. This api call addresses that. Sadly, the loop
ptr is not "reusable" across multiple calls to uv::run().

net complexity increase :/

- removing the remains of uvtmp.rs and rust_uvtmp.rs
- removing the displaced, low-level libuv bindings
in uv.rs and rust_uv.cpp

adds new c/c++ methods bound in rust for uvtmp::uv

as it stands, basic async nad timer support is added

the core impl is there, with a async handle in place
to take incoming operations from user code. No actual
uv handle/operations are implemented yet, though.

Closes #1909

Closes #1901