Add ccache plugin

[pileghoff]

Adds a simple, bare-bones ccache plugin to allow compilation caching using ccache without much fuzz. It works with custom test compiler, without the end user having to set it all up.

This is something we are using internally already, but i thought i might as well clean it up and upstream it (if you want it).

This is the first ruby i have ever written, so i don't expects that its perfect and welcome any feedback.