Kgio 2.0.0 - major API changes

kgio provides non-blocking I/O methods for Ruby without raising
exceptions on EAGAIN and EINPROGRESS. It is intended for use with the
Unicorn and Rainbows! Rack servers, but may be used by other
applications.

Changes:

This release should make Kgio easier and more consistent
to use across a variety of libraries/applications.

The global Kgio.wait_*able(=) accessor methods are gone in favor
of having default kgio_wait_readable and kgio_wait_writable
methods added to all Kgio-using classes. Sub-classes may (and
are encouraged to) redefine these if needed.