New version semantics: stable, oldstable, and testing.

This adds three special version names: stable, oldstable, and testing.

Examples:

    ccm create -v stable test # downlaods the latest stable version of C*, currently 2.0.9
    ccm create -v binary:stable test # akin to #136, this downloads a binary version of 2.0.9
    ccm create -v oldstable # Downloads the latest version of the previous major version, currently 1.2.18

    ccm create -v testing test # This downloads the latest testing version, currently 1.2.1-rc3

The last example isn't working because it appears there is no tarball for rc3 published yet, even though there's a tag. I think acceptable behaviour is for it to error our like it is rather than grab the previous version.