Make the version number PEP-440 compliant

Shuts up this warning:

  .../setuptools/dist.py:294: UserWarning: The version specified ('0.9.8dev (git a58d795)') is an invalid version, this may not work as expected with newer versions of setuptools, pip, and PyPI. Please see PEP 440 for more details.

src/pyspacewar/version.py

@@ -2,7 +2,7 @@
 PySpaceWar version number tracker.
 """
 
-version = "0.9.8dev"
+version = "0.9.8.dev0"
 
 
 def get_git_revision(unknown='', format=' (git %s)'):
@@ -21,5 +21,5 @@ def get_git_revision(unknown='', format=' (git %s)'):
 
 
 if 'dev' in version:
-    version += get_git_revision()
+    version += get_git_revision(format='+git.%s')