Add 'gcc' cookbook to install gcc 4.8 on ubuntu 12.04LTS

This cookbook currently provides two recipes:
 - `default.rb`, to install ubuntu 'gcc' default package
 - `ppa.rb`, to install latest 'gcc' from ubuntu-toolchain-r/test PPA

Address travis-ci/travis-ci#1379
Related to travis-ci/travis-ci#979

With attribute `as_default = true`, recipe `ppa` can install gcc 4.8 as default gcc compiler, which is good if we don't want to modify `travis-build` C/C++ builders (each time we update gcc). That said, I'm not sure if making gcc 4.8 default is 100% safe (won't it break anything on ubuntu 12.04?). An conservative option could be to create a specific VM image for C/C++ with gcc 4.8, but keep standard gcc package on other language VMs. @o11c @bendiken @mavam @maksbotan any thoughts on this point?