Feed gemspec into ExtensionTask so we get native.

At my office we have the need to pre-compile native gems prior to putting them on our gem server.  The rake-compiler gem provides this functionality but in order to use it you have to feed the gemspec into ExtensionTask.

More info here:
https://github.com/luislavena/rake-compiler#generating-native-rubygems

Example Usage:
  tedwards@*SNIP* ~/src/redcarpet-toddedw$ bundle exec rake native gem
  mkdir -p tmp/x86_64-linux/redcarpet/1.9.3
  cd tmp/x86_64-linux/redcarpet/1.9.3
  /nm/home/staff/tedwards/.rbenv/versions/1.9.3-p448/bin/ruby -I. ../../../../ext/redcarpet/extconf.rb
  creating Makefile
  cd -
  cd tmp/x86_64-linux/redcarpet/1.9.3
  gmake
  compiling ../../../../ext/redcarpet/autolink.c
  *SNIP*
  tedwards@*SNIP* ~/src/redcarpet-toddedw$ ls pkg/
  redcarpet-3.0.0-x86_64-linux  redcarpet-3.0.0-x86_64-linux.gem