Add the failing test from #27 as a testcase
keep ExtensinKey's C counterpart alive correctly

In keeping with our other ownership changes, it seems (at least, the testcase
provided in #27 makes it seem :) that we need to keep this around explicitly.
remove unneeded explicit lifetime

See https://groups.google.com/forum/#!topic/python-cffi/oe7Bd6klSZM for an
explanation:

> Btw, the "cffi_explicit_lifetimes[xcb_parts] = iov_base" line here is
> not needed, because the "xcb_parts" array will anyway be dead at the
> end of the function.  You just need to ensure that "iov_base" array
> remains valid until the end of the function too; it's enough to (but
> needed to) keep it in a local variable as you do.
Pass some extra buffer space to xcb_send_request

Thanks to Armin Rigo for debugging this one!

Closes #27
run nosetests with -v
fix failing test

We don't have a hardcoded list of all the atoms, and the interesting bit of
this test was to test the seg fault, so we can just get rid of this