Change version numbering for nightly and unstable

[peterfpeterson]

For both of these an incremental number does not make as much sense. Especially since currently the patch number is the number of commits since the last tag which occasionally resets on develop (goes backwards). The suggestion is to use [https://en.wikipedia.org/wiki/ISO_8601 ISO8601] strings to set the version. There are [http://www.cmake.org/Wiki/CMake_Useful_Variables#System_.26_Compiler_Information extra variables] to set in CMake. The will be set to:

• VERSION_MAJOR - same as current stable release
• VERSION_MINOR - same as current stable release
• VERSION_PATCH - date of build in GMT formatted as YYYYMMDD.HHMM

This would mean that last nights build would have the version "2.5.20130618.2303" (built Jun 19, 2013 12:03:25 AM at ISIS).

[peterfpeterson]

This issue was originally trac ticket 7321