Make plugin work with debugger-xml through UNIX sockets

This is a pretty big rewriting of the plugin.

I added support of the different printers to the 'debugger' gem, and
that allowed me to create new gem, 'debugger-xml', which is a 'debugger'
extension, and allows to display results not in plain text, but in XML
format, compatible with 'ruby-debug-ide' gem.

In debugger-xml, I added support of UNIX sockets, so now
vim-ruby-debugger doesn't need additional Ruby daemon
(bin/ruby_debugger.rb) for work anymore. Now, vim-ruby-debugger just
sends message through UNIX socket directly to debugger-xml, and
debugger-xml will poke vim-ruby-debugger when new event happens or
a command is handled and has something to output.

Thanks to all that stuff, vim-ruby-debugger works a bit faster now,
because I don't use TCP anymore, and because I got rid of one additional
hop (from vim-ruby-debugger to bin/ruby_debugger.rb), and also more
robust, since there is no additional daemon, and no need to handle
(i.e. start/stop) it anymore.

I also dropped Windows support, but I actually am not sure it worked
before - I even don't have Windows to check it out. If you still need
Windows support, you may try to use 'v1.0' branch, or contribute to
here. :)

Also, removed the requirement of 'lsof', debugger-xml now explicitly
tells its PID before daemonizing (I used to use lsof before to get PID of
bin/ruby_debugger.rb and rdebug-ide)