*: split out VERSION and set during build time

This patch creates a single canonical file to represent the version of the
repo (spec + tooling) at a particular point in time, so we do not have version
information in multiple locations that needs to be kept in sync. Build flags
are then used (specifically, '-ldflags -X') to inject this version into actool
and the ACE validator. Finally, the `+git` suffix is added to the current
version to reflect the real state of things and avoid confusion with the
actual 0.1.1 release.


/cc @philips