Tweaks to beta/alpha versioning

Adds a new 'beta cycle' variable that can be appended to the '-beta' version label, e.g. '-beta1'. Changes the version label for the beta channel temporarily to 'alpha'. Changes the artifact name of the beta channel to contain the version number instead of just being called 'beta'. The beta cycle number is currently set to 1.

The impact of this is that the first alphas will be called '1.0.0-alpha1' and the artifacts will also be called '1.0.0-alpha1-*.tar.gz'. We could alternately leave out the cycle number if we are confident there will be only one alpha cycle.

r? @alexcrichton cc @nikomatsakis @huonw