Add license_file to setup.cfg metadata (#609)

Without this, the LICENSE file is never included in the built wheels: this makes it harder for users to comply with the license.
With this addition a file LICENSE.txt will be created in the `xxx.dist-info` directory with the content of the `license_file` file, e.g. the top level LICENSE.
diff --git a/setup.cfg b/setup.cfg
index 058cdfc..166423e 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,6 +1,9 @@
 [wheel]
 universal = 1
 
+[metadata]
+license_file = LICENSE
+
 [aliases]
 release = egg_info -RDb ''