replace extension build tool make with rake to support platforms without make (e.g. windows)

Hi,

I had problems to install bond on windows+jruby without development kit: Although bond uses jrubys own readline, it creates a empty dummy makefile for jruby users. As windows has no make, installing the gem (even with the empty dummy make file) fails.

I switched from make to rake: as rake is installed by default with any ruby version, the rake command should be available on all systems. 

Alternatively, you could drop the c extension completely. I think it is only necessary for <1.9 ruby versions. As ruby 1.8 is no longer supported, there is no reason to keep the c extension.