COMP: Associate extension package name with the date of the last commit.

A similar problem was occurring while building the Slicer core and has
been fixed by r20028[1]. The associated issues were #1974 [2] and #1973 [3].

This commit associate to the package name the date of the latest commit
associated with the extension source repository. In case no repository
has been associated with the extension, the date "0000-00-00" will be
considered.

Thanks to "Chenxi Zhang <chenxizhang@fudan.edu.cn>" for reporting the
problem.

Associated discussion: http://slicer-devel.65872.n3.nabble.com/Problem-in-building-a-extension-tp4029816p4029857.html

Fixes #3356

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=20028
[2] http://www.na-mic.org/Bug/view.php?id=1973
[3] http://www.na-mic.org/Bug/view.php?id=1974