Fixed issue 119, no environment for the shell_exec and exec calls.

For each call exec or shell_exec, I have added the ability to prepend a
string. For example '/usr/bin/env -i '.

src/IDF/Scm/Svn.php

@@ -369,7 +369,7 @@ class IDF_Scm_Svn
      */
     public function getArchiveCommand($commit, $prefix='git-repo-dump/')
     {
-        throw new Exception(('Unsupported feature.'));
+        throw new Exception('Unsupported feature.');
     }