Added branch name back to command-line usage

I inadvertently removed the command-line usage of the tool.

This works now:
gitpuller [-h] git_url [branch_name] [repo_dir]

Author: sean-morris

nbgitpuller/pull.py

@@ -303,11 +303,13 @@ def main():
 
     parser = argparse.ArgumentParser(description='Synchronizes a github repository with a local repository.')
     parser.add_argument('git_url', help='Url of the repo to sync')
+    parser.add_argument('branch_name', default='master', help='Branch of repo to sync', nargs='?')
     parser.add_argument('repo_dir', default='.', help='Path to clone repo under', nargs='?')
     args = parser.parse_args()
 
     for line in GitPuller(
         args.git_url,
-        args.repo_dir
+        args.repo_dir,
+        branch=args.branch_name
     ).pull():
         print(line)