Add '--plain-version' commandline flag

Using this flag avoids generating a version string like
$UPSTREAM_VER+git.$TIMESTAMP.$COMMITHASH". Instead, the plain upstream
version is used.
Add unit tests and docstring
Rewrite spec file regardless of changes.

Allows to change version string format in spec file. Also drop
'store_const' from --plain-version flag. The service XML always wants
some text in tags, thus we have to use 'True' there, sigh.
Merge pull request #10 from saschpe/master-plain-ver

Add '--plain-version' commandline flag