Adds an `unreleased` flag to the version if not building from HEAD of a
tag. The compatibility check is bypassed if the client or server are
unreleased.

fixes #2110

This provides the Chart.yaml field `tillerVersion`, which is a semver
range. It allows users to choose to constrain a chart to a specific
version.

The reason for this is that we keep introducing new template functions,
but we have no way of saying "this chart will only work with Tiller
newer than...".

The check on version is _only_ done on Tiller. The client does not check
at all, since it does not do any template expansion on its own.