New fsl_generate_module_manifest command

For generating/updating a JSON manifest file. Some other enhancements too:

• Added ability to refer to 'built-in' manifests by aliases - the default manifest is referred to as fslcourse, and the Graduate Programme manifest as gradcourse, e.g. fsl_add_module -m gradcourse.
• Added support for a size field in manifest entries, specifying the file size in bytes.

fsl/add_module/tests/test_manifest.py

 #!/usr/bin/env python
 #
-# test_plugin_manifest.py -
+# test_manifest.py -
 #
 # Author: Paul McCarthy <pauldmccarthy@gmail.com>
 #
@@ -14,7 +14,7 @@ from unittest import mock
 
 from . import tempdir, touch
 
-import fsl.add_module.plugin_manifest as plgman
+import fsl.add_module.manifest as plgman
 
 
 def test_downloadManifest():