add gem:precompile task to create fat gems
fix for MinGW MSYS shell
change library file path to load
load pre-compile configs from config files

When create a pre-compiled gem in MinGW MSYS shell passing Ruby or OpenCV's paths,
the paths are converted to be invalid by MSYS shell.

E.g.
  When we run the following command
    $ rake gem:precompile RUBIES=C:/ruby19/bin/ruby.exe,C:/ruby20/bin/ruby.exe
  The following paths are expected
    ENV['RUBIES'] = "C:/ruby19/bin/ruby.exe,C:/ruby20/bin/ruby.exe"
  But we get the following
    ENV['RUBIES'] = "C;C:\\MinGW\\msys\\1.0\\ruby19\\bin\\ruby.exe,C;C:\\MinGW\\msys\\1.0\\ruby20\\bin\\ruby.exe"

This fix is for avoiding the problem.
update document
fix config file