Rebuild CRuby based building script without Makefile

All building script covert to CRuby based.
It's supported cross compiling feature.

Tested on CRuby 1.8.6 and 1.9.3

You can see building configuration in build_config.rb and gem spec files are in mrbgem.rake in doc/mrbgems/*.

@bovi: Could you update docs if matz accepted it. Sorry, my English is broken and you're perfect spreader.