add a version number

Version number is defined as a constant in sas.h.
Version number is in semantic format, meaning:

* The Patch number is incremented for bug fix releases.
* The Minor version number is incremented for releases that add new API
features, but are fully backwards compatible.
* The Major version number must be incremented for releases that break
backwards compatibility in any way.
Merge pull request #32 from Metaswitch/version-Number

add a version number